feat(cypher): string functions + fail loudly on unsupported syntax

Add single-argument string functions to projections — size, length, trim,
ltrim, rtrim, reverse — via the generic function recognizer.

More importantly, change the engine to FAIL LOUDLY on unsupported syntax instead
of silently projecting an empty column. An unknown function call (e.g.
split(...), coalesce(...)) or list indexing/slicing in RETURN/WITH now returns a
clear "unsupported function '<name>' (supported: ...)" error rather than a
valid-looking but blank result — the same silent-empty failure mode that hid the
labels() bug. This supersedes the #373 graceful-resync behaviour (its test is
flipped to assert the error; a new test covers an unknown function in RETURN).

Decision: full Cypher Tier 3 (lists/maps/paths/comprehensions/params — a value
data-model rewrite) is intentionally NOT implemented; clear errors on
unsupported features are the higher-value, lower-risk completeness win for the
read subset agents actually use.

Refs: Cypher read suite (tiers 1-2a + unsupported-syntax hardening)