perf(forge): compare from the recorded base while confirming the head

Status snapshots carry the base commit the host recorded with the head
(baseSha). ForgeService.diff passes it to the provider when the latest
status names the requested head, and forge-github starts the compare read
from it while its GraphQL read confirms the head; a stale head or another
base cancels and joins the early read.

packages/experimental/forge-github/README.md

@@ -46,7 +46,7 @@ Unfinished check runs and completed runs with stale, missing, or unrecognized co
 
 A pull request is `mergeReady` when it is open, not a draft, GitHub reports its merge state as `CLEAN`, `HAS_HOOKS`, or `UNSTABLE`, and the signed-in user has write, maintain, or admin permission. `mergeMethods` lists the squash, merge-commit, and rebase methods the repository allows, the user's default merge method first. Marking ready runs `gh pr ready <number> --repo <host>/<owner>/<name>`. Merging calls `PUT repos/<owner>/<name>/pulls/<number>/merge` through `gh api` with the method and the expected head SHA, so GitHub refuses once new commits arrived, which is reported as `stale`. Unlike `gh pr merge`, it never enables auto-merge, never joins a merge queue, and never deletes the head branch. A refused action reports GitHub's message, and a diagnostic that mentions authentication becomes `unauthenticated`.
 
-`diff` reads `headRefOid` and `baseRefOid` in one GraphQL query and returns `stale` when the head is not the requested commit, which must be a full 40- or 64-digit hexadecimal commit. Otherwise it reads `repos/<owner>/<name>/compare/<baseRefOid>...<headSha>` through `gh api --allow-escape-sequences` with the diff media type, the patch from the merge base of both commits, so neither end follows a moving branch. For every open and merged pull request measured, this patch is byte-identical to `gh pr diff`, which takes no commit argument. Unlike the pull request diff, the compare endpoint returns diffs over 300 files, so only output over `maxOutputBytes` or GitHub's own refusal makes such a diff `too-large`. `--allow-escape-sequences` keeps terminal escape sequences that file contents hold, which `gh` 2.97 and newer otherwise refuse to print; an older `gh` rejects the option with `unknown flag` before sending a request and prints such sequences unchanged, so that rejection repeats the read without it. `fileText` reads `repos/<owner>/<name>/contents/<path>?ref=<commit>` through `gh api` with the raw media type and accepts only a full 40- or 64-digit hexadecimal commit. `fileUrl` returns `https://<host>/<owner>/<name>/blob/<commit>/<path>`. Both paths percent-encode each segment. `commits` reads a pull request's commits through GraphQL, 100 per page with the owner, name, number, and cursor as variables, until `maxCommits`; every page must report the head and commit count of the first one and a cursor no earlier page returned, else the listing is `unavailable`. `compare` reads the patch from the merge base of two full commits to the second through the same compare request as `diff`. A refused read becomes `not-found` for HTTP 404, `unauthenticated` for a diagnostic that mentions authentication, `too-large` for GitHub's HTTP 406 refusal or `too_large` error, which a diff or file over its size limit draws, and `unavailable` otherwise, with GitHub's message. A read whose output exceeds `maxOutputBytes` becomes `too-large`, and one that outlives `timeoutMs` becomes `unavailable`.
+`diff` reads `headRefOid` and `baseRefOid` in one GraphQL query and returns `stale` when the head is not the requested commit, which must be a full 40- or 64-digit hexadecimal commit. Otherwise it reads `repos/<owner>/<name>/compare/<baseRefOid>...<headSha>` through `gh api --allow-escape-sequences` with the diff media type, the patch from the merge base of both commits, so neither end follows a moving branch. Each status snapshot carries `baseRefOid` as `baseSha`; given it, `diff` starts this comparison while the GraphQL query confirms the head, and a stale head or another base commit cancels the early comparison and waits for it to end before `diff` returns. For every open and merged pull request measured, this patch is byte-identical to `gh pr diff`, which takes no commit argument. Unlike the pull request diff, the compare endpoint returns diffs over 300 files, so only output over `maxOutputBytes` or GitHub's own refusal makes such a diff `too-large`. `--allow-escape-sequences` keeps terminal escape sequences that file contents hold, which `gh` 2.97 and newer otherwise refuse to print; an older `gh` rejects the option with `unknown flag` before sending a request and prints such sequences unchanged, so that rejection repeats the read without it. `fileText` reads `repos/<owner>/<name>/contents/<path>?ref=<commit>` through `gh api` with the raw media type and accepts only a full 40- or 64-digit hexadecimal commit. `fileUrl` returns `https://<host>/<owner>/<name>/blob/<commit>/<path>`. Both paths percent-encode each segment. `commits` reads a pull request's commits through GraphQL, 100 per page with the owner, name, number, and cursor as variables, until `maxCommits`; every page must report the head and commit count of the first one and a cursor no earlier page returned, else the listing is `unavailable`. `compare` reads the patch from the merge base of two full commits to the second through the same compare request as `diff`. A refused read becomes `not-found` for HTTP 404, `unauthenticated` for a diagnostic that mentions authentication, `too-large` for GitHub's HTTP 406 refusal or `too_large` error, which a diff or file over its size limit draws, and `unavailable` otherwise, with GitHub's message. A read whose output exceeds `maxOutputBytes` becomes `too-large`, and one that outlives `timeoutMs` becomes `unavailable`.
 
 Review submission checks the current pull request head, then posts a single inline comment or one completed review through the REST API with an explicit `commit_id`. Multiline comments carry their side and inclusive start and end lines. Review decisions are Comment, Approve, and Request changes. JSON request bodies travel on stdin. The provider never retries a mutation: a timeout, unexpected response, or server error after dispatch returns an unknown outcome that the caller must reconcile on GitHub. A commit arriving between the check and the write can make the explicitly addressed review outdated; GitHub does not offer an atomic head precondition on these endpoints.
 
@@ -100,7 +100,7 @@ None. Provider output reaches only host-side watchers.
 - **Batch deadline** — continuation pages of a pull request with many review threads lengthen its host's polling batch; a batch that outlives `ctx.forge`'s `fetchTimeoutMs` reports `unavailable` for every pull request in it, not only for the one with many threads.
 - **Rewritten base branch** — the diff starts from the merge base of the head and the base commit GitHub recorded at the pull request's last update. After the base branch is force-pushed, that merge base can differ from the one GitHub's pull request view uses, so the patch can include or omit commits until the pull request is updated.
 - **No fork discovery** — head lookups exclude pull requests whose head is in another repository.
-- **Rate limits** — every poll costs one GraphQL query per host, plus one query per further page of 100 review threads, and a complete check read one more query per further page of 100 check records; every diff read costs one GraphQL query and one REST request. All count against the signed-in account’s rate limit.
+- **Rate limits** — every poll costs one GraphQL query per host, plus one query per further page of 100 review threads, and a complete check read one more query per further page of 100 check records; every diff read costs one GraphQL query and one REST request, and a second REST request when the base commit changed after the latest status. All count against the signed-in account’s rate limit.
 - **No merge queue** — a base branch that requires a merge queue refuses a direct merge, and the provider never queues one.
 
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