release: stage v0.49.0 soak candidate

[odvcencio]

Scope

• Graduate the accumulated Unreleased entries into v0.49.0 dated 2026-08-04.
• Update the README release status and comparison links.
• Record the published v0.48.1 issue v0.48.0 regression: name field stamped onto separator commas (import_from_statement, import_statement, subscript) #660 patch.
• Add a Python regression guard for anonymous separator fields on both parser routes.

Verification

• Focused Docker test: TestIssue660AnonymousCommaHasNoField
• Docker release validator tests: go test ./cmd/releasegate -count=1
• git diff --check

Soak

After merge, dispatch the full ci.yml workflow with workflow_dispatch on the exact merge commit. Start the 48-hour soak only after that run succeeds. Keep v10 performance work outside this candidate.