Add value-only matrix snapshot bridge

[luisleo526]

What changed

• Add an opaque, reusable Snapshot API to PineMatrix, PineGenericMatrix<T>, and the bool specialization.
• Restore snapshots through copy-then-swap so a failed snapshot copy leaves the live matrix untouched.
• Cover valid, reassigned, na, and valid 0x0 receivers plus snapshot copy/move/reuse.

Why

The upcoming recursive calc-on-order-fills checkpoint work needs a stable matrix snapshot interface before matrix object-ID semantics are introduced. Landing this bridge separately keeps that later factorial factor narrow and independently testable.

This change is deliberately semantics-neutral: ordinary assignment and explicit .copy() retain their existing detached value behavior.

Validation

• Full engine build and CTest: 116/116 passed
• Full pinned corpus (62af2350917d..., REGEN=0): 252/252 built and ran, 0 failures
• Corpus grading: 247 Excellent / 4 Strong / 1 documented anomaly, zero tier drift versus the pinned authoritative report
• Cross-repo native compile using the candidate codegen matrix checkpoint traits: PASS
• Fresh read-only Grok review: no actionable findings

[Copilot]

Pull request overview

Adds a value-only “Snapshot” checkpoint/restore bridge for matrix types to support upcoming rollback/checkpoint work, while keeping existing matrix assignment and .copy() semantics unchanged.

Changes:

• Introduced an opaque Snapshot type plus snapshot() / restore() on PineMatrix, PineGenericMatrix<T>, and PineGenericMatrix<bool>.
• Implemented restore via copy-then-swap to keep the live matrix unchanged if snapshot copying throws.
• Added a new compatibility test covering snapshot copy/move/reuse, NA behavior, reassignment, and 0x0 matrices.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_matrix_snapshot_compat.cpp	New test exercising snapshot/restore semantics across matrix types.
tests/CMakeLists.txt	Registers the new snapshot compatibility test in the test suite.
src/matrix.cpp	Implements PineMatrix::snapshot() and PineMatrix::restore().
include/pineforge/matrix.hpp	Adds the public, opaque PineMatrix::Snapshot API and declarations.
include/pineforge/generic_matrix.hpp	Adds Snapshot + snapshot/restore for generic matrix templates and bool specialization.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.