fix(parser): const X = packed struct {} lost its name (cherry-pick of #3384)

[gHashTag]

Cherry-pick of #3384 onto the branch the website actually vendors.

Why it needed to come here separately

master and fix/struct-field-brace-nesting have diverged in both directions —
1327 commits on the branch that master lacks, 586 on master the branch
lacks. t27.ai vendors its corpus from this branch (1cd2877f9), so a fix
landed on master alone can never reach the site.

More usefully: master's corpus contains no packed struct at all, so master
could not test the fix. This branch is where specs/demos/hello_world.t27 lives —
the spec the Spec Explorer opens on — so this is the first place it was verified
end to end.

The conflict was real: the branch's union(enum) case occupies the slot the fix
used on master. Resolved by keeping both, side by side ahead of KwStruct.

Measured, same branch, same spec, only the parser differing

// before
parameter [31:0] Greeting = packed;
// struct
reg [7:0] _length;              // .length
reg signed [7:0] _trit;         // .trit

// after
// struct Greeting
reg [7:0] greeting_length;      // Greeting.length
reg signed [7:0] greeting_trit; // Greeting.trit

The struct was already being emitted — anonymously, because the declaration
was a ConstDecl. The fix removes the bogus parameter and restores the name.

What it does not do — measured, not assumed

yosys 0.63 synthesises both versions to 0 cells, 4 wires, 4 ports. It never
rejected the old output either; it printed:

Lexer warning: The SystemVerilog keyword `packed' is not recognized
unless read_verilog is called with -sv!

and carried on.

So this removes a warning and restores a lost name. It does not change
synthesis, and it was never a cause of the corpus's yosys failures. I speculated
otherwise two iterations ago; the refutation belongs next to the fix rather than
left standing in an older commit message. The module-shell / 0-LUT condition has a
different root cause — no hardware boundary — which the entry-points service
already measures at 387 specs.

Verification

• Suite green on this branch: 889 plus the per-crate suites, 0 failed
• FROZEN_HASH resealed with a shasum-computed digest, never transcribed
• Cargo.lock left alone: cargo build -p t27c prunes ~1000 lines from it, which
  is a build artifact and not a change

Closes #3383