config: migrate the display section to registry bindings

[berrym]

The display section kept its keys in both the legacy config_options[]
table and the CREG registry, with hand-written display_sync_to/from_runtime
hooks copying between them. config set wrote the struct field directly via
the legacy table, so a user's config set display.X was re-seeded the next
time a mode preset applied — it never reached the registry's SESSION layer.

This binds the eight dual-table display keys to their runtime cells and
removes their legacy rows, making the registry the sole writer:

• display.syntax_highlighting, display.autosuggestions,
  display.transient_prompt, display.theme_hot_reload,
  display.optimization_level, display.lle.pager.{enabled,min_lines,wrap_search}

config set, TOML load, and per-mode defaults now route through the registry
and write-through to the struct via the binding. A config set on a display
key — and the display lle … builtins, which already set the registry — now
lands in the SESSION layer and survives a mode switch (the previous
behavior re-seeded the user's value on the next mode preset).

display_sync_to/from_runtime are deleted and the section's hooks set to
NULL. display.performance_monitoring and display.ambiguous_width stay
legacy-only for now — neither is synced, so neither had the clobber; they are
their own migrate/wire backlog items.

Verified live: config set display.transient_prompt false survives a
lush → bash → lush round-trip; the per-mode defaults still resolve (lush on,
bash/zsh/posix off); config show display lists every key. Clean under
-Werror; 156/156 tests.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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