[Paffin]

Replace the per-commit full matrix and triple Rust runs with fast component feedback, explicit qualification and prebuilt release candidates.

• Ordinary PR/main runs select Linux unit/storage, SDK or Go checks from the diff; Web runs lint/types/unit/build only. Main compares against the last successful run so cancellation cannot drop pending changes.
• Full platform/backend/browser/security coverage runs nightly, manually and during candidate preparation. Linux coverage also supplies full-suite correctness; SDK runtimes share one checksum-verified server build.
• Cancel obsolete validation, cache compiled workspace crates (including failed iterations) and protect short checks from background concurrency groups.
• Pin each release candidate to its selected main commit so development can continue. Prepare archives/images/provenance in parallel with qualification; Release only validates/promotes the successful immutable candidate. Ready-only requests fail promptly instead of secretly compiling.
• Publication has a 3-minute job budget plus parallel 2-minute public-install checks. Cold preparation and runner queues are explicitly outside this five-minute ready-candidate target; no promise that arbitrary source compiles in five minutes.

Validation: all four new PR workflows passed; 807 selected Rust tests passed locally; workflow/candidate/request/pinning/cancellation tests, Python 3.9 compatibility, docs and actionlint passed. A full prepared-candidate rehearsal and timed no-write promotion will validate the new cross-workflow artifact path after merge. Existing v0.3.0-beta.7 is not republished.