docs(foundry): packs/ holds the manifest, not the audio

Provenance for a cut that shipped: which stem each loop came from, at what tempo, graded
how. The audio stays in Samples/ — this is the record, not a copy.

tools/foundry/packs/README.md

+# packs/ — the record of what was cut
+
+One directory per source pack, holding the **manifest and its views** for a cut that
+actually shipped:
+
+```
+packs/<pack>/
+  fred_kits.json     # the manifest — every sample, where it came from, its grade + tags
+  fred_kits.md       # the report: per-track probe verdicts, tempo, per-kit tables
+  fred_kits.tidal    # the cheat sheet — paste-ready, `n` index of every sample
+  kitcheck.md        # the audit
+```
+
+The audio itself lives in `~/Work/Sound/Samples/<kit>/` (symlinked into Dirt-Samples),
+never here — this is provenance, not a copy. The manifest is the record; `.md` and
+`.tidal` are views of it and can be rebuilt at any time with:
+
+```bash
+python3 -m engine.stempack --report-only --out packs/<pack>
+```
+
+Keeping the manifest is what makes a cut auditable months later: which stem a loop came
+from, at what tempo, graded how, and what CLAP heard in it.