refactor(synapse): extract embed-expansion strategy (OCP) from monolithic loop

[MarcoPorcellato]

Problem

_expand_macros_and_embeds_impl is a single while True loop handling block embeds, page embeds, cycle detection, and graph lookups. Each new embed type (macro, query, PDF) requires editing the core loop — Open/Closed Principle violation noted in docs/BUG_HUNT_REPORT.md §6.

Clean Architecture lens

Aspect	Assessment
Ring	Interface Adapter (synapse.py)
SOLID	OCP + SRP — one class/function per embed kind
Dependency rule	Strategies depend on LogseqGraph port, not KINETIC/CLI

Proposed Solution (incremental)

1. Define a small EmbedExpander protocol: match(text) -> span | None, expand(...) -> str
2. Implement BlockEmbedExpander, PageEmbedExpander with shared cycle context object
3. Keep _expand_macros_and_embeds as façade delegating to ordered strategies
4. Fix cycle duplication (#65) inside page strategy

Mentor scope

~4–6 hours. Good follow-up after #65 tests land.

Definition of Done

• No behavior regression (tests/test_synapse.py)
• make all passes

[MarcoPorcellato]

Shipped in Clean Architecture v1.6 working tree. make all passes (456 tests).