Insulate body-finding behind sp (API-containment increment 1)

[YLZha]

First increment of option 1 from the API-fragility review: make sp a deliberate insulation boundary so each Fusion-API quirk has one implementation site to maintain across version drift, instead of N copies in templates.

This increment: body-finding

Six templates each carried a private _all_bodies / _find_body loop over comp.bRepBodies + name matching — the exact pattern the audit flagged as fragile (wrong body picked after a mirror/pattern rename or proxy/native mismatch).

• Added standalone sp.bodies_in(comp), sp.find_body(name, comp), sp.find_bodies(pattern, comp) — thin wrappers over the already-canonical _util recursive helpers that DesignContext uses.
• Routed drawbore, tusk_tenon, splayed_legs, domino, scarf_joint, bed_rail_fastener through them. No raw .bRepBodies iteration remains in those files.

The fragile name-based identification now lives in one place — so the later hardening (UUID/feature association instead of name matching) is a single-site change.

Behavior-preserving by construction

• bodies_in returns the same shallow list as the old _all_bodies.
• find_body = _find_body_recursive (checks the component's own bodies first, then descendants — the same fallback the locals already delegated to).
• The 4 templates with local helpers keep their _find_body/_all_bodies signature (now one-line delegations) → zero call-site churn. scarf_joint/bed_rail had inline loops, replaced directly.

Verification

• py_compile clean on all 7 files; confirmed no raw .bRepBodies left in the migrated templates.
• The template Fusion fixtures (tests/test_template_drawbore.py, _splayed_legs, _domino, …) are the real regression guard — they run in Fusion and should be exercised there to confirm (I can't run them offline).

Next increments (separate PRs, same pattern)

• (B) addTwoPointRectangle → sp.sketch_rect* where parametric matters (skip throwaway cut-tool sketches)
• (C) hand-built Matrix3D/Move → an sp rotate/move shim
• (D) raw createForAssemblyContext → sp proxy helper (one place to add the .isValid guard)
• (E) raw participantBodies set → sp.ext_op

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