Complete Exception#backtrace: raise-time stack, qualified names, memoization

[sisshiki1969]

Summary

CRuby's Exception#backtrace reports the full live stack at raise time. monoruby only recorded the frames the exception actually unwound through (from the raise site down to the rescuing frame), so the backtrace of an exception raised and rescued in the same method was missing that frame's callers. This PR completes the backtrace to match CRuby while keeping the raise path cheap, per the design constraint that control-flow pseudo-exceptions (StopIteration, method-return, break) must not pay a backtrace-construction cost on every raise.

Approach

Rather than building a full backtrace eagerly at every raise, the exception keeps recording only the cheap (loc, sourceinfo, fid) tuples it unwinds through. The rest of the live stack is filled in once, lazily, at the catch point:

• Executor::complete_backtrace_for_rescue (executor.rs) is called from handle_error's rescue branch, just before take_ex_obj, while the caller frames are still live. It walks the rescuing frame's callers via each inner frame's saved call-site pc — the same mechanism as Kernel#caller — and appends their tuples. No string formatting happens here; that stays lazy in #backtrace. Control-flow pseudo-errors (MethodReturn / BlockBreak / Throw) never reach this path, so they pay nothing.

• Fully-qualified frame owners (store.rs func_description): frames now render the owner's qualified name (Ns::Cx.foo, not Cx.foo), special-casing Object so top-level methods still read as Object#foo.

• #backtrace memoization (exception.rs): the built Array is memoized in the /backtrace hidden ivar, so repeated calls return the same mutable object (e.backtrace.unshift(x) is visible on the next call; e.dup.backtrace is distinct). set_backtrace writes the same ivar, unifying the explicit store with the memo.

• #backtrace_locations decoupled from the string backtrace via a new __raise_backtrace intrinsic that returns only the raise-time capture. This makes set_backtrace(strings) on a never-raised exception keep #backtrace_locations nil, and an Array of Locations sets both (CRuby 3.4+).

Test plan

• Fixes core/exception backtrace_spec, backtrace_locations_spec, and set_backtrace_spec (all now 0 failures against CRuby 4.0.2).
• Adds tests/backtrace.rs — 6 differential tests: rescued-in-same-method, propagated chain, qualified class name, memoized/mutable array, set_backtrace(strings) leaving locations nil, and unraised → nil.
• Full lib suite regression: 1830 passed, 0 failed.

Remaining core/exception failures are out of scope: interrupt_spec (3, real SIGINT delivery / signaled exit status) and top_level_spec (2, cause-chain printed to STDERR + custom backtrace via the 3rd raise argument).

🤖 Generated with Claude Code

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Generated by Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 94.91525% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.49%. Comparing base (2423ce0) to head (fc295dc).

Files with missing lines	Patch %	Lines
monoruby/src/executor.rs	93.93%	2 Missing ⚠️
monoruby/src/builtins/exception.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #896   +/-   ##
=======================================
  Coverage   90.49%   90.49%           
=======================================
  Files         189      189           
  Lines      123170   123227   +57     
=======================================
+ Hits       111461   111515   +54     
- Misses      11709    11712    +3     

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