feat: prompt rules for lossless round-trips (0.3.1)

Two coherence rules added to DEFAULT_SYSTEM_PROMPT:
- different values of the same type get distinct placeholders (context),
  e.g. old/new IBAN -> [PER_1.IBAN:Ancien] / [PER_1.IBAN:Nouveau]
- never absorb adjacent punctuation/separators into a placeholder

Took the live Gemma-4 (Infomaniak) e-learning demo from 4/5 to 5/5 exact
round-trips. 36 tests passing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -4,6 +4,16 @@ All notable changes to this project are documented here. The format is based on
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/), and this project adheres to
 [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.3.1] - Unreleased
+
+### Changed
+
+- Default prompt hardening for lossless round-trips: (1) two **different** values of the same type never
+  share a placeholder — they must be distinguished by context (e.g. `[PER_1.IBAN:Ancien]` vs
+  `[PER_1.IBAN:Nouveau]`), preventing a within-message collision (old/new IBAN); (2) the model must not
+  absorb adjacent **punctuation/separators** (commas, spaces, parentheses) into a placeholder. Together
+  these took the live Gemma-4 e-learning demo from 4/5 to 5/5 exact round-trips.
+
 ## [0.3.0] - Unreleased
 
 ### Changed