docs/tests: reserve Phase A/B/C/D for the per-writer recovery flow

Three terminologies were calling themselves Phase A/B in PR #72:

1. Per-writer recovery (canonical, four phases A/B/C/D — sidecar /
   commit_staged loop / manifest publish / sidecar delete in
   `docs/runs.md:157`).
2. Per-table staged-write contract from MR-793 (two phases —
   `stage_*` then `commit_staged`).
3. Test-narrative scaffolding (Phase A = setup the failure, Phase B
   = verify recovery — used as section dividers in failpoints.rs).

Same letters, three meanings; three reviewers including the bots have
already misread the code in the resulting fog. This change keeps
"Phase A/B/C/D" exclusively for #1 and rewrites the other two:

- `ensure_indices_phase_a_btree_failure_leaves_existing_tables_writable`
  → `ensure_indices_stage_btree_failure_leaves_existing_tables_writable`
  (matches the `stage_create_btree_index` API verb).
- Comment at `table_ops.rs:610` and the test docstring at
  `failpoints.rs:807` rewrite "a Phase A failure in the staged-index
  path" → "a stage-step failure in the staged-index path".
- Twelve `// Phase A:` / `// Phase B:` test scaffolding comment
  headers in `failpoints.rs` (across six test fns) become
  `// Setup:` / `// Recovery:`.
- A "Phase letter convention" note added near `docs/runs.md:165`
  spells the rule out for future readers.

Also bundled: rename
`composite_flow_init_load_branch_merge_time_travel_optimize_cleanup`
→ `composite_flow_canonical_lifecycle` so it pairs as a story name
with `composite_flow_multi_branch_sequential_merges` (the previously-
deferred symmetry rename).

No behaviour change. Both renamed tests pass; full failpoints (18) +
composite_flow (2) suites pass; workspace baseline + clippy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/runs.md

@@ -163,6 +163,15 @@ are left at `Lance HEAD = manifest_pinned + 1`.
 3. **Phase C**: publisher commits the manifest.
 4. **Phase D**: writer deletes the sidecar.
 
+> **Phase letter convention.** Throughout the recovery code, log
+> messages, failpoint names (e.g. `branch_merge.post_phase_b_pre_manifest_commit`),
+> and the per-writer integration tests, "Phase A/B/C/D" refers
+> exclusively to the four-step lifecycle above. The per-table
+> staged-write contract (`stage_*` then `commit_staged`, two steps)
+> is referred to by those API verbs — never by phase letters — so a
+> reader of `recovery.rs`, `failpoints.rs`, or this document only
+> encounters phase letters in the per-writer context.
+
 A failure between Phase A and Phase D leaves the sidecar on disk. The
 next `Omnigraph::open` (gated on `OpenMode::ReadWrite`) runs the
 recovery sweep in `crates/omnigraph/src/db/manifest/recovery.rs`: