feat: add anchor placement validation

[zharinov]

Summary

• Add semantic validation for anchor (.) placement
• Boundary anchors at sequence start/end require parent named node context
• Interior anchors (between items) are always valid
• Add AnchorWithoutContext diagnostic with helpful hint

Valid:

(parent . (first))     ; ✓ boundary anchor inside named node
{(a) . (b)}            ; ✓ interior anchor (between items)

Invalid:

{. (a)}                ; ✗ boundary anchor without parent context

Test plan

• cargo test -p plotnik-lib anchors passes (16 tests)
• Updated parser tests to use valid anchor patterns