B5: build-time FW_VERSION from git describe #187

_{Recreated from Strycher/Field_Compass-archive#187 — originally opened by @Strycher on 2026-08-09. Credentials redacted where present.}

Epic: #154

Inject FW_VERSION at build time via -D from git describe, replacing the hand-maintained #define FW_VERSION "0.51.1" at Field_Compass.ino:28.

The drift this closes

FW_VERSION reads 0.51.1; the newest tag is v0.51.10. Nine tags were cut on chore commits with no firmware change, against CLAUDE.md's rule that the two must match. The device's /diags reports 0.51.1 today.

Constraint - SAFELANE section 7

Do not reintroduce an auto-commit version bumper. The 2026-03-29 incident was exactly that: a workflow committing version bumps that rebase merges then orphaned, leaving the version stale for 270 commits while tags advanced. Compute at build time; commit nothing.

Acceptance criteria

• /diags and the About screen report a git describe derived version
• A dirty working tree is visibly marked (-dirty), not silently reported as clean
• No workflow commits a version change