Add missing settings to an existing config file

[TerrifiedBug]

The template a new install gets lists every setting — that is how a config-only feature like tap_to_toggle is meant to be discovered. But ensureFileExists returns early when the file is already there, so anyone who upgrades keeps a config with no line for anything added since. "Edit config…" opens a file that hides half the settings, and the README becomes the only way to learn they exist.

The daemon now backfills missing defaults at startup, and again immediately before "Edit config…" opens the file.

What it will and will not do

• Values you have set are untouched. Only keys absent from the file are added.
• Unknown keys are left alone — your own additions, or keys from a newer yap.
• Your formatting survives: insertion is textual, not a re-serialize, so nothing but the new lines moves. Missing keys land where the template lists them, not where the alphabet puts them.
• It declines rather than forcing it. A section written on one line has nowhere to put a line, and splicing one in turns a tidy hand-written config into a mess that happens to parse. Those files keep their formatting and their missing keys.
• Every rewrite is verified before it lands: the new text has to parse, and it has to differ from what was there by exactly the defaults being added. Anything else and the file is left as it is. Losing someone's config to a clever edit is far worse than an out-of-date one.

Verification

Six fixtures, each run through the real daemon under a faux HOME (CFFIXED_USER_HOME), so nothing touched a real config:

fixture	result
stale real-shape config, only tap_to_toggle missing	key inserted in template position, every value and every line of formatting intact
no dictation object at all	whole section written, pretty-printed at the right indent, top-level keys in template order
compact one-liner { "dictation": { "hotkey": "right_option" }, … }	byte-identical — declined, as designed
unknown keys (my_own_note, dictation.future_key)	both survive; missing defaults added around them
malformed JSON	untouched; still warns the way it always did
already complete	never written — same mtime

Also two cosmetic fixes to Foundation's pretty printer so a generated section reads like the template next to it: "key" : value → "key": value, and slashes are no longer escaped (~\/Recordings → ~/Recordings). Neither can corrupt anything, because the result is parsed and compared before it is written.

No measured path changes: one file read and parse at daemon start, and a write only when a key is actually missing.