fix(msl): reject >32,767 fragments/entry on write instead of silent overflow

[trishorts]

Problem

MslPrecursorRecord.FragmentCount is a signed 16-bit field, but MslWriter narrows the fragment count with an unchecked (short) cast. An entry with more than 32,767 fragment ions silently wraps to a negative count on write, producing a file that throws OverflowException (negative array length) only later, on read — i.e. silent corruption at write time, surfaced far from the cause.

This is reachable in practice: with internal-ion annotation (a supported .msl feature), the fragment count grows roughly quadratically with peptide length, so a proteoform of ~250+ residues annotated with internal ions can exceed 32,767 fragments.

Fix

Validate on write. MslWriter now throws a clear, actionable ArgumentException that names the offending entry and the limit, on both the streaming and in-memory write paths, instead of emitting a corrupt file. No on-disk format change.

Verification

• 32,767 fragments/entry → writes and round-trips OK (unchanged).
• 32,768 fragments/entry → now throws on write:
  Entry '…' has 32768 fragment ions, which exceeds the .msl per-entry limit of 32767 (FragmentCount is a 16-bit field in MslPrecursorRecord). …
  (Previously: silent wrap on write → OverflowException on read.)

Scope

This is the safe, non-breaking guard against silent data corruption. Lifting the limit entirely (widening FragmentCount to int32) is a versioned format change and is intentionally out of scope here.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.34%. Comparing base (a730eaa) to head (ac8100a).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
+ Coverage   82.33%   82.34%   +0.01%     
==========================================
  Files         382      382              
  Lines       50509    50518       +9     
  Branches     6102     6103       +1     
==========================================
+ Hits        41586    41599      +13     
+ Misses       7734     7729       -5     
- Partials     1189     1190       +1     

Files with missing lines	Coverage Δ
mzLib/Readers/SpectralLibrary/MslWriter.cs	96.01% <100.00%> (+0.59%)	⬆️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.