Guard conditional switch discriminants in optimized Ok()

[AaronWebster]

Problem

The optimized Ok() switch (introduced in #253, extended in #256/#257) reads its
discriminant once and emits, as a safety net:

const auto emboss_reserved_switch_discrim = <discriminant>;
if (!emboss_reserved_switch_discrim.Known()) return false;
switch (emboss_reserved_switch_discrim.ValueOrDefault()) { ... }

When the discriminant is itself a conditionally-present field and is read from a
buffer too short to contain it, the discriminant reads as Unknown, so this guard
makes Ok() return false — even though the message is valid, because the fields
gated on the (absent) discriminant don't exist.

This is latent on master: no existing test schema produces the shape, and the only
surviving guard in the goldens (parameters.emb's Axis) is on a bare arm, which is
sound.

Root cause

The guard is sound iff the group has at least one bare arm (existence condition
exactly discriminant == K): then an Unknown discriminant leaves that field's
has_X() Unknown, so the unoptimized Ok() bails for the same reason. When every
arm carries a residual (extra conjuncts beyond discriminant == K), a valid message can
have an out-of-bounds (Unknown) discriminant while every arm's residual is statically
false — making every has_X() Known-false (absent). The blanket guard wrongly rejects it.

Fix

In _emit_switch_block, classify each group:

1. Provably-present discriminant → switch, no guard (unchanged, Drop redundant Known() guard when discriminant is provably Known #257).
2. Not provably-present, ≥1 bare arm → switch + Known() guard (unchanged — sound).
3. Not provably-present, all arms have residuals → guarded switch: dispatch only when
   the discriminant is in bounds, otherwise fall back to per-field has_X().Known() checks.
   (An Unknown discriminant can never make has_X() Known-true, so the value check is
   unnecessary in the fallback.)

Cases 1 & 2 are untouched, so every existing golden is byte-for-byte identical; only the
new structs add codegen.

Tests

Adds two structs to testdata/condition.emb:

• ResidualConditionalDiscriminant — the bug (all-residual arms on a conditional discriminant).
• BareConditionalDiscriminant — the sound-guard case (bare arms).

with regression tests in condition_test.cc covering: discriminant absent (valid, was
wrongly rejected), discriminant present-but-truncated (invalid), gated field present/truncated,
and discriminant matching no arm. The tests pass under both the optimized and unoptimized
(_no_opts) Ok() implementations, proving the optimized path now matches ground truth.

The same fix + tests will be propagated to the open stacked PRs (#258, #259) so the bug cannot
be reintroduced as they land.