fix(qec): correct PyMatching merged-edge mapping and realtime observable output

[vedika-saravanan]

Description

Fix two PyMatching issues in the realtime decoding path:

• Make parallel-edge error output map back to the H-column retained by the configured merge strategy.
• Pass the observable matrix to top-level realtime PyMatching construction so it uses native decode_to_obs output instead of decoding to error bits followed by observable projection.

Adds regression coverage for keep_original, independent, smallest_weight, and replace, plus realtime observable-output construction.

Runtime / performance impact

GB200, surface code d=7 / r=7, 1,000 decode-triggering shots per run:

Metric	Before	After
PyMatching decoder_call average	5.09–5.47 µs	3.35–3.70 µs
Final syndrome arrival to corrections ready average	13.78–14.09 µs	12.09–12.31 µs

All three post-change server runs passed with zero residual logical errors.

Self-review checklist

Please confirm each item before requesting review. Check [x] or strike
through and explain.

Before requesting review

• I reviewed my own full diff in GitHub or my editor.
• PR is in Draft if it is not yet ready for review.
• Temporary / debugging changes have been removed.
• Local test logs reviewed; no unexplained warnings or errors.
• CI logs reviewed; no unexplained warnings or errors.
• Full CI has been run.

Scope and size

• PR is under ~1000 lines, or an exception is justified in the description.
• Refactoring-only changes are isolated in their own PR(s).
• No existing tests were disabled or modified just to make this PR pass
  (if so, an issue has been raised).

Tests

• New functionality has new tests.
• Tests fail if the new functionality is broken (including crashes), not
  just when it is missing.
• Negative tests added where exceptions are expected. No new exception path.
• Truth data added where simple EXPECT_* / assert checks are
  insufficient for algorithmic correctness. The small explicit graph has exact expected outputs.
• CI runtime impact considered; team notified if significant.

Documentation

• Public-facing APIs have Doxygen docs. No public API was added or changed.
• User-visible behavior changes have public docs, or a follow-up is
  tracked. This is an internal correctness and performance fix.

Code style

• Naming follows the existing convention (snake_case vs camelCase) for
  the area being modified.

Dependencies

• No new third-party dependencies, or the team has been notified and
  OSRB tickets filed.