fix: prevent elname SV leak when Start/End handlers die

[toddr-bot]

What

Fix memory leak of element name SVs when Start or End handlers throw exceptions.

Why

When a Start handler dies, call_sv longjmps past av_push(cbv->context, elname) — the element name SV (refcount 1) is never stored or freed. Similarly in endElement, SvREFCNT_dec(elname) is skipped. Every handler exception leaks one SV. This adds up in applications that parse untrusted XML and use handlers that validate/reject content by dying.

How

Use SAVEFREESV(elname) inside the ENTER/LEAVE scope that wraps the call_sv. This registers the SV for automatic cleanup if the scope unwinds via exception. On the normal (non-exception) path, SvREFCNT_inc_simple_void(elname) before LEAVE preserves the SV for the post-scope operations (av_push in startElement, SvREFCNT_dec in endElement).

Audited all 19 XS callbacks — startElement and endElement are the only ones with non-mortal SVs across a call_sv boundary. All other callbacks properly use sv_2mortal before pushing SVs.

Testing

• Added t/handler_die.t with 6 tests: Start/End die propagation, parser reuse after exception, and stress test (10 consecutive exception cycles)
• Full test suite passes

🤖 Generated with Claude Code

---

Quality Report

Changes: 2 files changed, 91 insertions(+), 1 deletion(-)

Code scan: clean

Tests: passed (OK)

Branch hygiene: clean

Generated by Kōan post-mission quality pipeline

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.63%. Comparing base (45216a7) to head (103a592).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
Expat/Expat.xs	50.00%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
- Coverage   75.73%   75.63%   -0.10%     
==========================================
  Files           1        1              
  Lines        1092     1096       +4     
  Branches      342      344       +2     
==========================================
+ Hits          827      829       +2     
  Misses         59       59              
- Partials      206      208       +2     

Flag	Coverage Δ
perl	75.63% <50.00%> (-0.10%)	⬇️
xs	75.63% <50.00%> (-0.10%)	⬇️

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