Merge Phase Ε: the .rsbank bank package — pure codec, filesystem shell, both verbs, and the frozen compatibility corpus

# Conflicts:
#	docs/COMPLETED.md

docs/product/bank-package.md

@@ -381,7 +381,7 @@ trim, the seed is the literal `Imported bank`.
 
 **The probe.** Let `seed` be that string and `fold(x)` be `BankBook`'s own uniqueness
 key — strip leading/trailing ASCII whitespace, lower-case ASCII letters
-(`bank_book.h:252-258`). Take the **first** name in this sequence whose fold is not
+(`bank_book.h:263-269`). Take the **first** name in this sequence whose fold is not
 already carried by a bank in the destination book:
 
     seed,  seed + " 2",  seed + " 3",  seed + " 4",  …
@@ -410,7 +410,7 @@ implementations diverge:**
    `B + 1` candidates is free by pigeonhole, so no cap is needed and none should be
    added.
 4. **The fold has exactly one home.** `import_plan` must **not** re-implement
-   `nameKey` — `bank_book.h:252-258` says in as many words that a drifted second copy
+   `nameKey` — `bank_book.h:263-269` says in as many words that a drifted second copy
    would let the uniqueness invariant be violated. The probe therefore runs behind
    `BankBook`'s own folding, which means Ε-W2-T2 adds **one additive public `const`
    member** to `BankBook` (recommended: `std::string uniqueDisplayName(const
@@ -690,7 +690,7 @@ constructors.
   freshly-generated pair.
 - **Bank generation.** Import mutates bank content that live ReaSampler 9000
   instances may play, so it must `bumpBankGeneration()`
-  (`src/shell/persist/session.h:108`, whose own comment says call sites "err toward
+  (`src/shell/persist/session.h:121`, whose own comment says call sites "err toward
   bumping"). Export mutates nothing and must bump nothing, write no ext state, and
   open no undo point.
 - **Beta/stable channel isolation.** Packages are channel-**agnostic** and this is
@@ -725,6 +725,30 @@ contemplates one untracked capture, an import strands hundreds).
 
 ---
 
+## Implementation decisions — Ε-W2-T1
+
+Not [Daniel]-class forks — both were `[propose at review]` calls in `docs/PLAN.md`'s
+Ε-W2-T1 track, answered at implementation review rather than by Daniel, and recorded
+here per this phase's own convention for keeping such answers where the design lives
+rather than only in the track's own now-stale open-questions line.
+
+- **Affordance: both the bindable action and the panel row.** The action targets the
+  **active** bank and is the only spelling that can reach the **pool** (the panel's
+  `showTabMenu` returns early on `isPool()` — a named-bank-tab context menu has no tab
+  to right-click for the pool), while the exported unit's own definition above includes
+  the pool. The panel row is the direct gesture on a specific named bank. Neither
+  subsumes the other.
+- **Default file name: the bank's display name**, sanitized through
+  `capture_paths::sanitizeStem`, seeded into `<projectDir>/<stem>.rsbank`. A
+  project-derived name was the rejected alternative: three banks exported from one
+  project must produce three distinguishable files, and a project-derived name
+  collides on the second export. Known wart, worth recording rather than hiding:
+  `sanitizeStem` collapses an all-non-ASCII display name to the literal `capture`, so
+  two such banks still collide — the existing rename verb is the recovery, same as the
+  import-side auto-suffix collisions above.
+
+---
+
 ## Non-goals and guardrails
 
 - **No auto-insertion of imported audio into the arrange.** Same rule as capture.