In the source

For developers. Everything above is implemented under src/formats/chk/ and src/formats/mpq/, with the file-level policy in src/editor/:

File	Does
`src/formats/mpq/scm.ts`	Opens and writes the archive over mopaq: finds `staredit\scenario.chk`, accepts a bare `.chk`, reports how the member was stored, keeps the other members.
`src/formats/chk/reader.ts`	Parses the chunk stream into a `ChkFile`: every occurrence in order, truncation and negative lengths recorded, trailing bytes kept. `layer()` reproduces the game's overwrite semantics.
`src/formats/chk/sections/registry.ts`	One entry per known section: combine mode, fixed size (some depend on the map's dimensions), record stride, and the `editorOnly` flag the Save dialog's strip groups are built from.
`src/formats/chk/scenario.ts`	The `Scenario` model, `parseScenario` and `serializeScenario`, the `dirty` set that decides what is re-encoded, the revision table, `revisionSections` for the settings pairs, and the insertion order for a section the file did not have.
`src/formats/chk/sections/*.ts`	The codecs: `terrain`, `objects` (units, sprites, doodads, locations), `players`, `strings`, `settings`, `triggers`, `cuwp`, `sounds`, and the `vcod` table.
`src/formats/chk/create.ts`	File ▸ New, and `requiredSections(version)`, which Check Map tests a file against.
`src/editor/save.ts`	The save plan: strip groups, merging repeats with the registry's mode, the sector size per compression, and the warnings the dialog shows.
`src/editor/mapBuild.ts`, `src/services/mapBuild.ts`	Built maps: packing the step's output with the source beside it, restoring the source on open, and running the steps without letting one cost a save.
`src/editor/sections.ts`	Section-level reads and writes for the Section Explorer plugin and `api.document.sections`.
`src/editor/validate.ts`	Check Map.
`src/data/triggerDefs.ts`, `src/formats/triggers/text.ts`	Which field of a trigger record holds which argument, and the text format.

The rules that matter when changing any of it:

Any code that mutates the scenario has to mark every section it touched dirty, or the change never reaches the file.
To model a new section: add a codec under sections/, decode it in parseScenario, add a case to encodeSection, add it to MODELLED_SECTIONS, and add a round trip to tests/chk.test.ts.
Optional sections are null in the model when the file had none, and the encoder omits them then. Do not write a zeroed one.
The tests re-encode Blizzard's own maps byte for byte: tests/chk.test.ts, tests/save.test.ts, tests/settings.test.ts, tests/data-settings.test.ts, tests/trigger.test.ts, tests/briefing.test.ts and tests/cuwp.test.ts run against fixtures/maps/*.scx, which is gitignored (Blizzard data) and skipped when absent. A green run on a clone without the fixtures has not exercised them.