allow unreachable, and expand when to use panic

[ShahakShama]

Note

Low Risk
Low risk: documentation-only changes with no runtime or API impact. Main risk is behavioral inconsistency if teams interpret the new panic!/unreachable! guidance differently.

Overview
Clarifies the error-handling conventions in style.md by expanding when panic! is appropriate (unrecoverable states vs. bugs) and explicitly allowing unreachable! for provably unreachable code paths while discouraging unimplemented!.

Renames and broadens the messaging guideline section to cover unreachable! (and invariant-style expect) messages, emphasizing documenting why a failure is impossible rather than restating the error.

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[ShahakShama]

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• allow unreachable, and expand when to use panic #13636 👈 (View in Graphite)
• add section when to return result #13635
• error types should have a Error suffix #13627
• add section on expect vs allow #13626
• delete combinators section #13416
• add section on test names #13276
• make short unit test into a recommendation #13275
• improve documentation section #13274
• remove track_caller sentence #13267
• remove sections of re-export #13266
• add logging section #13261 : 1 other dependent PR (#13262 )
• add instruction that TODOs should have an owner #13260
• add section advocating to use Self #13259
• add section on american english vs british english #13257
• remove section about private fields altogether #13166
• unite getter and setter sections, and modify setter section #13165
• reorder API sections (no text change) #13164
• add section saying that code inside macro should be formatted #13163
• add section on future returning pending #13162
• improve ai section #13147
• improve casting section #13146
• make the example in combinators compile #13145
• add comment in return value section to split checking the value and performing action to 2 lines #13144
• remove bad example in foo_or_else section #13141
• improve the starting paragraphs of testing section #13139
• improve macros section #13137
• remove unnecessary misc sections #13131
• reorder sections (move AI and Testing up) #13118
• remove open issues section #13115
• improve external dependencies section #13114
• remove github smell testing section #13113
• create misc section, move stuff into it, and order by importance (no erasure of anything yet) #13112
• change file layout section #13078
• add section on sleep in tests #13073
• add section on dependency injection #13072
• move expect message to a separate section with more explanation #13071
• add directory layout section #12950
• rename allocations and add &Option<T> #12781
• remove laziness  #11591
• rewrite types section #11588
• more comments  #11587
• more comments  #11586
• more comments  #11585
• more comments  #11584
• arithmetic: remove ditto for other mathematical operations. #11848
• gilad/style-guide

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