text: add pb.text.decode and wire it into conformance dispatch

Hand-written recursive-descent parser for the textproto grammar
covering every bucket the proto3 conformance suite exercises:
decimal/hex/octal integer literals with full 32/64-bit range checks,
float specials (inf/infinity/nan any case, oversize exponents
saturating to ±inf, underflows to ±0), C-style + \u/\U string escapes
with adjacent-literal concat and surrogate rejection, aggregate {} /
<> bodies, repeated short-form `[a, b, c]`, `key: K value: V` map
entries, the `[type.googleapis.com/...]` inline Any form alongside
the direct `type_url:`/`value:` form, enum-by-name-or-number,
reserved-name silent drop, numeric-field-ID tolerance, and
duplicate-singular-field rejection.

Plugin gains a small reserved_names emitter so the parser can match
mainline TextFormat::Parser's "silently drop reserved" rule. The Any
WKT descriptor advertises its real fields (type_url + string,
value + bytes) so the generic body walker can populate it directly
when the input doesn't use the inline-URL form.

cmd/conformance/core.lua stops short-circuiting text_payload to
`skipped` and runs it through pb.text.decode. The proto3 TextFormat
input suite climbs from 8 ✓ / 426 skipped to 406 ✓ / 18 skipped / 10
expected failures. The 10 surviving failures all share one cause
(proto3 -0.0 elision in the codec, not a parser bug — documented in
test/conformance/known_failures_text.txt). Binary+JSON conformance
holds at 1478 ✓. 591 unit tests pass across both codegen modes.

Closes the text-conformance-output branch.