docs(comments): sweep em dashes from source comments, test prose, and manifest

Same treatment for the prose the compiler carries: doc and line comments
across src/ and tests/, test assertion and error-message strings (prose
surfaces: the tunnel-map registration/establish errors, the key-source
decode payload, assertion messages), scripts/matrix.sh comment lines,
the fuzz sidecar's comments and README (corpus seeds untouched), and the
Cargo.toml comment headers plus the crates.io description. The AGENTS.md
no-em-dash rule (convention 14) now holds for the whole tracked tree.

Verification: repo-wide git grep is clean (single exempt hit: the
verification command's own search pattern), cargo test (18 suites, zero
failures, default and per-feature builds via scripts/matrix.sh), clippy
--all-targets -D warnings and clippy all-features, fmt --check, doc
--no-deps, and the fuzz corpus gate (fuzz/shared, 11 tests) all green.

scripts/matrix.sh

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 # The decline-matrix runner (tasks/implementation/w5-decline-matrix.md):
-# the per-build verification set as one command — `cargo test` per
+# the per-build verification set as one command: `cargo test` per
 # feature build (none / git / session / tunnel / sftp-only /
 # --all-features; the sftp-only run is the no-op-feature fact itself
 # verified), clippy ×2 (-D warnings, with and without