Make start() / end() callable as boundary functions (reserved-keyword lexer/grammar work)

[zzylol]

Summary

Bare start() / end() parse-fail (start() → invalid promql query), while foo @ start() works. Prometheus exposes start() / end() as functions returning the query range boundaries as scalars.

Current state (verified)

Already documented as deferred in the asap commit c51beaf that added the other experimental functions:

start()/end() are NOT added: start/end are reserved lexer keywords for the @ modifier, so a table entry can't make them callable (needs lexer/grammar work, deferred).

src/parser/function.rs on asap has 0 entries for start / end; GreptimeTeam upstream functions.rs likewise.

Work

• Let the lexer / grammar treat start / end as callable in function-call position while keeping them valid as @ modifiers (the reserved-keyword collision the commit note describes).
• Function-table entries returning a scalar.

Corpus impact

6 rejections. Small, but distinct lexer/grammar work — not a table-only addition.

Constraint

All changes on the asap branch; no upstream operations.