c-accel: arch prereqs — compat contract, C-side strategy, build scaffolding

Three companion specs under docs/specs/ formalize the boundaries
established in docs/c-accel.md, unblocking bd-mq7 (descriptor → C
plan compiler):

* c_accel_compat.md (bd-47e) — pinpoints what must stay byte-equal
  between PB_ENABLE_C unset and =1: public surface, generated
  module wrappers, 64-bit cdata, WKT shapes, unknown fields,
  extensions, errors. Calls out the lazy-view exclusion.

* c_accel_strategy.md (bd-z7x) — pb_plan struct layout, field-name
  luaL_ref caching, 4 KB stack-backed pb_buf, cached per-field
  stack indices (the 2× win from spike Phase B), sub-buffer over
  backpatching, map/oneof/unknown handling.

* c_accel_build_packaging.md (bd-wky) — where the C module lives
  (runtime/pb/c/), how it builds, what the rockspec gains, the CI
  matrix shape.

Scaffolding that lands now:

* runtime/pb/init.lua — PB_ENABLE_C=1 opt-in pcall hook; the
  loaded module (or nil) is exposed as pb.c_runtime for
  introspection. Silent fallback when the module is absent.

* Justfile — `build-c` / `clean-c` recipes (stub erroring cleanly
  until bd-ra6 lands runtime/pb/c/), new lua_cpath constant,
  LUA_CPATH wired through `test` and `test-one`.

* .builds/{pure-lua,c-enabled}.yml — sourcehut CI manifests, one
  per activation mode (sourcehut has no matrix; parallel jobs go
  in separate files). ubuntu/noble images.

* .sourcehut/conformance.yml — outside .builds/ so it doesn't
  auto-submit; trigger manually with `hut builds submit` before
  releases.

* .gitignore — runtime/pb/c_runtime.{so,dylib} and runtime/pb/c/*.o.

745/745 tests pass with PB_ENABLE_C unset and PB_ENABLE_C=1
(silent fallback verified).

Closes bd-47e, bd-z7x, bd-wky. Unblocks bd-mq7.