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/tests/fixtures.ts

@@ -54,6 +54,7 @@ export function pullRequestNode(number: number, overrides: Record<string, unknow
     headRefName: `topic-${number}`,
     headRefOid: 'f'.repeat(40),
     baseRefName: 'master',
+    baseRefOid: 'b'.repeat(40),
     additions: 128,
     deletions: 14,
     mergeable: 'MERGEABLE',

packages/experimental/forge-github/tests/graphql.spec.ts

@@ -56,6 +56,7 @@ describe('snapshot mapping', () => {
       headBranch: 'topic-1',
       headSha: 'f'.repeat(40),
       baseBranch: 'master',
+      baseSha: 'b'.repeat(40),
       additions: 128,
       deletions: 14,
       mergeability: 'mergeable',

packages/experimental/forge-github/tests/provider.spec.ts

@@ -1,5 +1,5 @@
 import { changeRequestKey } from '@deepseek-ai/dsh-experimental-forge'
-import { describe, expect, it } from 'vitest'
+import { describe, expect, it, vi } from 'vitest'
 import { GhOutputLimitError, GhTimeoutError, GhUnavailableError } from '../src/gh.ts'
 import type { GhOutput, GhRunner } from '../src/gh.ts'
 import { PULL_REQUEST_COMMITS_QUERY, THREADS_QUERY } from '../src/graphql.ts'
@@ -446,6 +446,55 @@ describe('GitHub provider reads', () => {
     expect(gh.calls[1]?.at(-1)).toBe(`repos/octo/demo/compare/${base}...${head.toUpperCase()}`)
   })
 
+  it('starts the comparison from the base the status recorded while it confirms the head', async () => {
+    const confirmed = Promise.withResolvers<GhOutput>()
+    const gh = fakeGh(args => args.includes(`query=${PULL_REQUEST_COMMITS_QUERY}`)
+      ? confirmed.promise : output('diff --git a/x b/x\n'))
+
+    const read = provider(gh.run).diff(pull(7), head, signal, base)
+    await vi.waitFor(() => { expect(gh.calls).toHaveLength(2) })
+    expect(gh.calls.map(call => call.at(-1))).toContain(`repos/octo/demo/compare/${base}...${head}`)
+    confirmed.resolve(output({ data: { repository: { pullRequest: { headRefOid: head, baseRefOid: base.toUpperCase() } } } }))
+
+    await expect(read).resolves.toBe('diff --git a/x b/x\n')
+    expect(gh.calls).toHaveLength(2)
+  })
+
+  it('cancels the early comparison and waits for it when the head moved, the base differs, or the head read fails', async () => {
+    const moved = 'e'.repeat(40)
+    for (const [answer, expected, compares] of [
+      [output({ data: { repository: { pullRequest: { headRefOid: pushed, baseRefOid: base } } } }),
+        { code: 'stale', message: `the pull request head moved from ${head} to ${pushed}` }, [base]],
+      [output({ data: { repository: { pullRequest: { headRefOid: head, baseRefOid: moved } } } }), 'diff --git a/y b/y\n', [base, moved]],
+      [output('', 1, 'gh: Something went wrong (HTTP 502)'), { code: 'unavailable', message: 'gh: Something went wrong (HTTP 502)' }, [base]],
+    ] as const) {
+      const calls: string[][] = []
+      const cancelled: string[] = []
+      const run: GhRunner = async (args, operation) => {
+        calls.push([...args])
+        if (args.includes(`query=${PULL_REQUEST_COMMITS_QUERY}`)) return answer
+        const range = String(args.at(-1))
+        if (range.includes(`${moved}...`)) return output('diff --git a/y b/y\n')
+        // The early comparison runs until the provider cancels it.
+        await new Promise((resolve) => { operation.addEventListener('abort', resolve, { once: true }) })
+        cancelled.push(range)
+        throw operation.reason
+      }
+
+      await expect(provider(run).diff(pull(7), head, signal, base)).resolves.toEqual(expected)
+      expect(calls.map(call => String(call.at(-1))).filter(target => target.includes('/compare/'))
+        .map(target => target.split('/compare/')[1]?.split('...')[0])).toEqual(compares)
+      expect(cancelled).toEqual([`repos/octo/demo/compare/${base}...${head}`])
+    }
+  })
+
+  it('refuses a base that is not a full commit object name before starting gh', async () => {
+    const gh = pinned()
+
+    await expect(provider(gh.run).diff(pull(7), head, signal, 'HEAD~1')).rejects.toThrow('forge-github: invalid base commit "HEAD~1"')
+    expect(gh.calls).toEqual([])
+  })
+
   it('reports a head that moved past the requested commit as stale without reading the diff', async () => {
     const gh = pinned({ head: pushed, base })