perf(parser): append sorted intervals without scanning the set

[zahariash]

This work was produced with the help of language models.

Fixes #1301. Part of #1290.

The first query in a process deserialized the lexer's ATN, which builds large Unicode character sets one interval at a time, and IntervalSet::add walked the whole interval list for each one: about half of the first query's 9.39 ms compile time for RETURN 1 (query-summary time, median of 8 processes). The 0.18 ms in #1290 is a later query.

Fix. An interval that goes into an empty set, or starts after the last interval without touching it, is appended directly; any other interval takes the existing loop, so the resulting set is the same. This is a local patch to the vendored runtime (ANTLR 4.13.1, marked in the source); upstream 4.13.2 and the dev branch have the same loop.

Performance. First query compile 9.39 → 5.16 ms, 77.7M → 42.0M instructions; later queries unchanged. Mallocs are unchanged too: about 68.4K for the first query (within run-to-run noise) and 601 per later RETURN 1 at one thread. The rest of the first compile is ANTLR filling its prediction cache on first use.

Tests. AntlrIntervalSetTest.AddKeepsCanonicalForm: random and sorted inserts plus touching intervals, against brute-force membership and the canonical sorted, disjoint, non-adjacent form. EofMergesOnlyWithTouchingRange: EOF (-1) merges with a range starting at 0 and stays separate from one starting at 1. EmptyIntervalAndRangeLimits: an empty interval is ignored, and appends and merges near SSIZE_MAX give the same set. All three fail if the fast path appends an interval that touches the last one. Full ctest on this branch: 3103/3103.

Independent of #1297 and #1300; with all three the first query compiles in 5.1 ms and executes in 0.33 ms (query-summary median of 8 processes).