M3: offline-render capture spike (master-mix / time-selection)

ICaptureBackend/CaptureRequest seam + OfflineRenderBackend driving snapshotted
RENDER_* with dither/normalize forced off for bit-identical output, RenderFailed
via filesystem check, and a pure capture_paths lib with tests. Spike action registered.

src/capture_paths.h

@@ -0,0 +1,56 @@
+#pragma once
+// capture_paths — the REAPER-free path arithmetic behind offline capture.
+//
+// PURE MODULE (CLAUDE.md §load-bearing split): NO REAPER types, NO SWELL, NO
+// vendor/ includes. Standard library only. The capture shell resolves the
+// current project directory via REAPER APIs, then hands the raw strings here so
+// the fiddly, easy-to-get-wrong path arithmetic (bank subfolder, unique file
+// name, absolute render dir, project-relative index path) is unit-tested outside
+// the DAW.
+//
+// Path convention: this module works in forward-slash form and does NOT touch
+// the filesystem. The bank subfolder name is a fixed constant so the same
+// project always resolves the same bank location (determinism).
+
+#include <string>
+
+namespace reasampler {
+
+// The project-relative bank subfolder. All captured wavs live here so the bank
+// travels with the .rpp (CONTEXT.md §Settled decisions: per-project bank).
+inline constexpr const char* kBankSubfolder = "reasampler_bank";
+
+// A resolved pair of paths for one capture: where REAPER must be told to write
+// (absolute, because RENDER_FILE wants a directory REAPER can create/open) and
+// what we store in the BankIndex (project-relative, because the index is
+// relative-paths-only — CLAUDE.md precision invariant).
+struct BankPaths {
+    std::string absoluteDir;    // <projectDir>/reasampler_bank  (forward slash)
+    std::string relativePath;   // reasampler_bank/<fileName>    (index value)
+    std::string fileName;       // <stem>.wav                    (full file name)
+    std::string fileStem;       // <stem>                        (RENDER_PATTERN — REAPER appends the extension)
+};
+
+// Normalizes a path to forward slashes and strips any trailing slash. Empty in
+// -> empty out. Pure string transform (does not consult the filesystem).
+std::string normalizeSlashes(const std::string& path);
+
+// Sanitizes a caller-supplied base name into a filesystem-safe stem: keeps
+// [A-Za-z0-9._-], replaces every other byte (spaces, slashes, quotes, control)
+// with '_', and collapses to "capture" if nothing usable remains. Deterministic:
+// the same input always yields the same stem (feeds bit-identical file naming).
+std::string sanitizeStem(const std::string& baseName);
+
+// Derives the bank paths for one capture.
+//   projectDir : absolute directory of the current .rpp (any slash style)
+//   baseName   : human base for the file stem (sanitized)
+//   uniqueTag  : caller-supplied disambiguator appended to the stem (e.g. a
+//                timestamp or counter) so repeated captures do not collide.
+//                Also sanitized. May be empty.
+// Produces "<stem>[_<tag>].wav". The relativePath is always project-relative and
+// forward-slashed so it satisfies BankIndex::add's relative-only invariant.
+BankPaths deriveBankPaths(const std::string& projectDir,
+                          const std::string& baseName,
+                          const std::string& uniqueTag);
+
+} // namespace reasampler