bugfix(15): normalise apply_chat_template's BatchEncoding (transformers 5.x)

apply_chat_template returns a BatchEncoding ({input_ids: [...]}) on transformers
>= 5 where 4.x returned a bare list[int]. build_masked_example treated the render
as a dict, so len/slicing were wrong and the prefix-differencing spuriously
raised "chat template is not additive" on every real model. Extract the id
sequence via a _render_ids helper; verified the assistant-only mask against
mistralai/Mistral-7B-Instruct-v0.2. The fake tokenizer returned a bare list and
missed this, so a BatchEncoding-returning variant now guards it.

CHANGELOG.md

@@ -7,6 +7,18 @@ are documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and the project follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [1.0.1] - 2026-06-18
+
+### Fixed
+- `build_masked_example` could not derive the assistant mask on transformers
+  ≥ 5: `apply_chat_template` now returns a `BatchEncoding` (`{input_ids: [...]}`)
+  where 4.x returned a bare `list[int]`, so the render was treated as a dict and
+  the prefix-differencing spuriously raised "chat template is not additive" on
+  every real model. The id sequence is now extracted either way; verified the
+  assistant-only mask against `mistralai/Mistral-7B-Instruct-v0.2`. The
+  fake-tokenizer test gained a `BatchEncoding`-returning variant so this can't
+  regress.
+
 ## [1.0.0] - 2026-06-18
 
 First release: the training and evaluation pipeline that turns posix-sdc
@@ -38,4 +50,5 @@ trajectories into a fine-tuned shell operator.
   mypy-strict codebase; an optional `[gpu]` extra (torch / transformers / peft);
   and a dependency on `posix-sdc[hub]`. Released under GPL-2.0.
 
+[1.0.1]: https://git.code.tiararodney.com/tiara/sekft/compare/v1.0.0...v1.0.1
 [1.0.0]: https://git.code.tiararodney.com/tiara/sekft/releases/tag/v1.0.0