feat(stovepipe): derive build strategy in process step

[mnoah1]

What?

• Add source-control-backed build-strategy selection to Stovepipe’s process stage.
• Pin admitted requests to an incremental last-green baseline when ancestry is valid; otherwise use a full build.
• Fall back safely to full builds when the baseline is unavailable, and return unexpected source-control errors for normal consumer handling.
• Add coverage for cold starts, rewrites, unavailable ancestry, failures, and queue CAS reloads.

Why?

Implementing proposed strategy selection from https://github.com/uber/submitqueue/blob/main/doc/rfc/stovepipe/steps/process.md

Test Plan

Incremental:

• Seed an existing queue value in the DB matching the fake source control head
• call uber.submitqueue.stovepipe.Stovepipe/Ingest
• Check request in DB:

state:          processing
build_strategy: incremental_since_green
base_uri:       git://manual/incremental/HEAD

New Request:

• Empty/fresh DB
• call uber.submitqueue.stovepipe.Stovepipe/Ingest
• Check request in DB

state:          processing
build_strategy: full
base_uri:      <blank>

Not an Ancestor

• Seed an existing queue value in the DB that does not match the fake source control (so ancestor check returns false)
• Empty/fresh DB
• call uber.submitqueue.stovepipe.Stovepipe/Ingest
• Check request in DB

state:          processing
build_strategy: full
base_uri:      <blank>

• Planning some more integration tests around this flow

[behinddwalls]

A few non-blocking nits from review — the core derive→claim→persist ordering looks right (strategy is derived against the same queue-row version the claim CAS validates, so the persisted baseline can't be stale), and threading strategy/baseURI through markProcessing also fixes the latent reload-clobber on main. Nits inline.