Use no-PCH sharding for Linux Clang 19 Main builds

[flip-engineer]

Summary

Linux x86_64 Main builds with Clang 19 use the qualified no-PCH sharder by default. The build retains baton2.c, the aggregate object, the executable, six shard objects, and a hash-bound receipt. The receipt records the resolved compiler path and binary hash, version probe, per-command arguments and completion status, elapsed time, and stdout/stderr digests. Other platforms and compiler versions use the serial build route.

The retained Main verifier reruns the deterministic splitter, checks all six generated source hashes and retained object hashes, validates compiler and command records, and checks the linked outputs. The handoff transfers and verifies the six shard objects so a later retained build can repeat those checks. Legacy handoffs keep their existing three-phase and three-artifact requirements.

Validation

• A remote C-only build compiled the immutable 60,136,705-byte input (8b3f622adf9e471455889e2313d86b33af8ef03cc5ff63cc886ac8abcf2d4e7b) with the current driver and verifier. All six shard compiles, aggregation, and link completed naturally with exit status 0. Build receipt: /var/tmp/baton-pr808-f9786fa-c-only/out/baton2.build.json, SHA-256 09d2c2a0b5eb5fa897589982266bac6bcb7e49dcf707644f0f8c8e2e3a23c56a. The executable SHA-256 is 2a41ab5a7a04535c94c11a2fa81ed19df069faed02f908207447cf78ee075d60.
• The direct build verifier rederived the split and validated all six retained objects. Transfer validation passed for the six-object handoff bundle; bundle SHA-256 b1a5817cae3bcb85ea4462bb14009cadb84e4fe99e2bc69983477d08a89cc3d5.
• The remote pinned reuse fixture passed 32 cases after adding extraction-path relocation for the split, compile, aggregate, and link paths. Result: /tmp/reuse-combined-fixture.21_dzb02/results.json, SHA-256 4917af5749f0aecbd127cbf7a009f25f6e624ec36160fe4270b1cce8f7cad351.
• git diff --check passed. No checks are reported for current head ccb7fce4757d8d374dd94ea8717b098f3acbbe38. The prior run 39010029551 ended with exit 143 in both Linux jobs and contained no compiler or source diagnostic.