support {{ }} brace escaping in string literals

[kacy]

summary

literal braces in string literals previously required chr(123)/chr(125),
because a { always opened interpolation. this adds the universal doubling
convention: {{ emits a literal { and }} emits a literal }, while a
single {expr} still interpolates as before.

print("plain {{literal}} braces")   # -> plain {literal} braces
print("hello {name}")               # -> hello world
print("json-ish {{\"k\": 1}}")      # -> json-ish {"k": 1}

two coordinated changes:

• the lexer skips {{ pairs when scanning a string literal to decide whether
  it interpolates (and inside trailing segments of an interpolated string)
• the ir consumer collapses {{/}} to single braces in the string table —
  the same decode pass that already handles \n, \t, \\, etc.

a single { still means interpolation, so existing code is unaffected
(a lone literal { never worked before anyway).

the regenerated self-host/ir_driver is included so the change takes effect
on the native build path, matching how prior frontend changes are committed.

what was tested

• new regression tests/cases/test_brace_escaping.pith covering plain {{}},
  interpolation, mixed literal+interp, lone }}, json-ish, and empty {}
• full Cranelift regression suite (91 passed, 0 failed)
• self-hosted examples (83) and self-hosted regressions — all passing
• bootstrap fixed-point verification

design notes

}} needs no lexer change: a single } is already literal everywhere outside
an interpolation expression, so doubled close braces survive into the string
table and are collapsed there alongside {{.