t27c seal --save writes one of a spec seal files, and can twin it by case #3418

Two defects in one place, and both were found by being bitten rather than by review.

1. A spec owns more than one seal, --save writes one

Measured over .trinity/seals:

The pattern is a naming scheme that changed: <Module>.json beside <dir>_<Module>.json.

Refreshing 116 stale seals through this command in #3416 reached 58 and stopped for exactly this reason, and the remaining 58 had to be rewritten by hand. It bit again one pass later in #3417: editing four specs produced ten stale seals, and --save would have fixed four.

The duplicates are not deletable. bootstrap/src/math_compare.rs opens .trinity/seals/PellisFormulas.json by its bare name, and that file is one of a pair with physics_PellisFormulas.json. So the tool has to maintain them.

93 specs also carry a seal whose name is not an identifier at all — specs/tri/utils/logger.t27 has "[]const u8".json, utils_"[]const u8".json and utils_TriLogger.json. A seal named after a type string. Left alone here.

2. The derived name can differ from the tracked file only in case

seal_file_path computes <parent>_<module>.json. For specs/ar/restraint.t27 that is ar_restraint.json, while the tracked file is ar_Restraint.json.

macOS resolves those to one file and hides it. On the case-sensitive filesystem CI runs on, --save creates a second seal for the same spec, and neither looks wrong.

The test for the second half cannot fail on macOS, and says so

Verified by mutation: making the lookup case-sensitive leaves the test green here, because the filesystem is case-insensitive — creating Aa then aA yields one file. The test now probes the filesystem and skips loudly rather than reporting a pass it did not earn:

SKIP save_writes_into_an_existing_case_variant: this filesystem is case-insensitive,
so the assertion cannot distinguish the guard from its absence

It discriminates on Linux, which is where CI runs. The other test — that a stale duplicate is refreshed — does fail on the mutant here.