docs: design proposal for user-overridable ESP sdkconfig

[zackees]

Summary

Design proposal capturing how a fbuild user should be able to override
ESP-IDF `CONFIG_*` settings without rebuilding the framework, what
surfaces should exist, and how they layer.

No code change. Just `docs/sdkconfig.md` (288 lines) plus an
INDEX.md row and a docs/CLAUDE.md cross-link.

What's proposed

A 5-layer precedence chain (lowest → highest):

1. Framework pre-baked `sdkconfig.h` (baseline, current behavior)
2. `fbuild.toml` `[sdkconfig]` table (future fbuild-native config)
3. `sdkconfig.fragment` in project root (recommended primary surface, Kconfig syntax)
4. `platformio.ini` `board_build.esp-idf.sdkconfig_options` (PIO compat)
5. `platformio.ini` `build_flags = -D CONFIG_*` (highest, existing PIO compat)

Merged dict emits as `-D CONFIG_*` flags (correctness) AND as a
synthesized debug `.fbuild/build//sdkconfig.h` (visibility, not
on the include path).

The current 4-key `SdkConfigSummary` boolean parser becomes a thin
wrapper around the expanded `SdkConfig` dict — no behavior change for
the existing `.eh_frame` policy consumer.

What it explicitly rejects

• User-owned `sdkconfig.h` in project tree (footgun: framework header
  has hundreds of keys, partial override → undefined behavior)
• "Just expose build_flags = -D and call it done" (pollutes .ini, no
  Kconfig copy-paste path, no provenance, no fbuild.toml migration
  story)

Open questions (called out in doc)

1. Fragment filename: `sdkconfig.fragment` vs `.local` vs `.user`
2. Per-environment fragments (`sdkconfig..fragment`)?
3. Unknown-key diagnostics — warn or stay silent for forward-compat?
4. Cache invalidation surface for the synth header fingerprint

Test plan

• No code change → no tests required
• INDEX.md picks up the new doc (3 new FAQ rows)
• docs/CLAUDE.md cross-link added in the "Other docs" list

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

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📥 Commits

Reviewing files that changed from the base of the PR and between 357629c and 8b11efc.

📒 Files selected for processing (3)

• docs/CLAUDE.md
• docs/INDEX.md
• docs/sdkconfig.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/sdkconfig-design

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