fix(synapse): unify unresolved embed macro replacement semantics

[MarcoPorcellato]

Bug Description

SYNAPSE embed expansion treats unresolved targets inconsistently:

Macro type	Missing target behavior
{{embed [[Page]]}}	Replaced with empty string (macro swallowed)
{{embed ((uuid))}}	Left unchanged in output

Downstream RAG sees silent content loss in one case and raw Logseq macros in the other.

Clean Architecture lens

Aspect	Assessment
Ring	Interface Adapter (synapse.py)
SOLID	Liskov / consistency — same conceptual operation (embed resolve) should share failure semantics
Uncle Bob	Clean Code: symmetric error paths; pick one policy and document it

Steps to Reproduce

# Missing page
"- {{embed [[NoSuchPage]]}}\n"  → ''

# Missing block uuid
"- {{embed ((00000000-0000-0000-0000-000000000000))}}\n"  → '{{embed ((...))}}'

Expected Behavior

Documented, symmetric policy, e.g.:

• Always strip macro and insert "", or
• Always leave a visible sentinel like [unresolved embed]

Suggested fix

• Align block + page branches in _expand_macros_and_embeds_impl
• Add parametrized tests in tests/test_synapse.py
• Note behavior in docs/ARCHITECTURE.md SYNAPSE section

Context

Local code study wave 10 runtime probe.

[MarcoPorcellato]

Runtime probe on main (wave 11): both unresolved {{embed [[Page]]}} and {{embed ((uuid))}} are replaced with empty string today — output 'p b' for - p {{embed [[NoPage]]}} b. If the original report expected block macros to remain in output, that behavior is not reproducible on current main; consider reframing as documented policy (empty vs sentinel) rather than asymmetric bug, or close if empty-string is the intended contract.