Implement the 5 empty function bodies in specs/tri/collections/stack.t27

[gHashTag]

Closes #3753

Opened from the bee branch queen-3753, which was pushed but never turned into a pull request. Every changed path is inside the boundary the issue declares:

• specs/tri/collections/stack.t27

The acceptance criteria are structural and were checked by the swarm, not by a human; the review-side oracle is what decides whether the generated code compiles and passes its tests.

[gHashTag]

Closing: master does not need this branch, measured with a t27c built from master at 1d5529b.

$ t27c spec-status specs/tri/collections/stack.t27
IMPLEMENTED
$ t27c gen specs/tri/collections/stack.t27 > /tmp/t27-gen.zig && grep -c 'not yet implemented' /tmp/t27-gen.zig
0
$ grep -cE '^[[:space:]]*test[[:space:]]+("|[A-Za-z_])' specs/tri/collections/stack.t27
10

This branch's own version of the file reads IMPLEMENTED with 0 stub(s) and 10 test(s). The pull request could not merge either way - mergeStateStatus: DIRTY, and a conflicted branch gets no required check, which is why it showed as waiting for CI rather than as needing a rebase.

Nineteen conflicted bee pull requests were compared this way; the six that carried something master needed are in #4289.