HS: count-filtered events + first modifier (+5 tests)

Parser: parse-on-feat now consumes `first` keyword before event-name (sets
count-min/max to 1) and a count expression after event-name — `N` (single),
`N to M` (range), `N and on` (unbounded above). Number tokens are coerced
via parse-number. Emits :count-filter {"min" N "max" M | -1} part.

Compiler: scan-on threads count-filter-info; the handler binding wraps the
fn body in a let-bound __hs-count counter. Each event fire increments the
counter and (when count is in range) executes the original body. Each
on-clause registers an independent handler with its own counter, so
`on click 1 ... on click 2 ... on click 3` produces three handlers that
fire on their respective Nth click (mix-ranges test).

Generator: dropped 5 cluster-34 tests from skip-list — `can filter events
based on count`, `... count range`, `... unbounded count range`, `can mix
ranges`, `on first click fires only once`.

hs-upstream-on: 43/70 → 48/70. Smoke 0-195 unchanged at 172/195.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/playwright/generate-sx-tests.py

@@ -110,10 +110,6 @@ SKIP_TEST_NAMES = {
     "can pick event properties out by name",
     "can be in a top level script tag",
     "multiple event handlers at a time are allowed to execute with the every keyword",
-    "can filter events based on count",
-    "can filter events based on count range",
-    "can filter events based on unbounded count range",
-    "can mix ranges",
     "each behavior installation has its own event queue",
     "can catch exceptions thrown in js functions",
     "can catch exceptions thrown in hyperscript functions",
@@ -129,7 +125,6 @@ SKIP_TEST_NAMES = {
     "can ignore when target doesn't exist",
     "can ignore when target doesn\\'t exist",
     "can handle an or after a from clause",
-    "on first click fires only once",
     "supports \"elsewhere\" modifier",
     "supports \"from elsewhere\" modifier",
     # upstream 'def' category — namespaced def + dynamic `me` inside callee