build: inject BuildTime + Commit ldflags into the mobile binary

[myleshorton]

What

Extend EXTRA_LDFLAGS (already carrying common.Version) with:

• common.BuildTime — UTC RFC3339 build timestamp
• common.Commit — short git SHA

Both flow into the existing android/macOS/iOS gomobile-bind targets via EXTRA_LDFLAGS. Windows-safe (powershell branch for the timestamp).

Why

Pairs with getlantern/radiance (getlantern/radiance#556) to make a shipped binary self-describing. A 9.1.14 TestFlight build linked a stale, pre-fix keepcurrent despite go.mod declaring the fix; a build-time + commit stamp (with radiance now logging linked dep versions) makes such a stale build obvious in logs and crash reports instead of silent.

🤖 Generated with Claude Code

https://claude.ai/code/session_01WGpKfgkbfdJwwsTaouMDoR

Summary by CodeRabbit

• New Features
  • Builds now include additional metadata, such as build time and commit information, alongside the version number.

[Copilot]

Pull request overview

This PR extends the existing Go linker flag injection for mobile builds so the produced binaries carry additional build-identifying metadata (build timestamp and git commit), helping correlate shipped artifacts with source and dependency state.

Changes:

• Adds a UTC RFC3339 BUILD_TIME stamp (with a PowerShell branch for Windows).
• Adds a short GIT_COMMIT stamp and injects both into EXTRA_LDFLAGS alongside the existing common.Version.

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 3fcce485-8050-4b1e-9676-88b7d9d6e3ef

📥 Commits

Reviewing files that changed from the base of the PR and between f954364 and bd2d4e1.

📒 Files selected for processing (1)

• Makefile

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📝 Walkthrough

Walkthrough

The Makefile's default EXTRA_LDFLAGS was expanded to stamp common.BuildTime and common.Commit in addition to common.Version. New variables BUILD_TIME (platform-dependent) and GIT_COMMIT (git short hash) were introduced to supply these values.

Changes

Build ldflags stamping

Layer / File(s)	Summary
Add build metadata variables and update ldflags Makefile	Adds BUILD_TIME and GIT_COMMIT variables and updates EXTRA_LDFLAGS to stamp common.Version, common.BuildTime, and common.Commit via linker flags.

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately reflects the main change: adding BuildTime and Commit ldflags to the mobile build.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fisk/build-stamp

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