M1: xpj container and ordered JSON tree

Branch: m1-xpj-tree → main

What changed

New Go module git.monoexp.com/bajdev/mpckit (go 1.24).
New package internal/xpj (no Wails, no domain knowledge), per build plan 4.1:
- Container (file.go): gunzip, split off the five header lines, parse the body; write header + body, gzip level 6 with mtime 0, XFL 0, OS 3 (the same 10 header bytes the device writes). ReadFile / WriteFile, Data(), Clone().
- Ordered tree (node.go, json.go): parsed token by token with json.Decoder + UseNumber(). Objects keep key order (and duplicate keys); numbers keep their original text. Written in the device layout: one key or value per line, no indentation, ": " after keys, [] / {} for empty containers, no trailing newline, < > & / not escaped.
- New values: NewInt (4), NewFloat (always 96.0, never 96; Python-repr shortest form, matching the reference writer), NewFloat32 (rounds to float32 first: 12.4 → 12.399999618530273), plus string/bool/null/array/object.
- Helpers: Get / Set by path (tracks[0].program.drum.instruments[4].layersv[0]), Lookup / SetKey / DeleteKey, array Insert / Append / Remove, deep Clone, Int() / Float() accessors.
Moved demo-projects/ to testdata/device/ (build plan section 3 and 10.1); updated the two doc references.

Only the decompressed bytes are guaranteed identical. The compressed bytes differ from the device's (different deflate implementation); the device accepted the Python writer's gzip output, which uses the same header bytes as ours.
String escaping follows the reference writer (Python json, ensure_ascii=False). None of the device files contain an escape, so the device's own escaping style is unknown.
Writing a NaN/Inf float panics, and a Number node with invalid text fails at write time rather than producing a bad file.

go vet ./... and go test ./... pass.

M1 done-when: TestRoundTripDeviceProjects reads DemoBlank, Demo02 and DemoFile and re-encodes each byte-identical to the decompressed original (about 3 MB each), both directly and through gzip and back.
The same passes for the device-accepted files in testdata/accepted (T1-T3) and testdata/wav-formats (W1, W2).
Shape check on DemoBlank: tested header, 66 data keys, data.version 28, 128 pads, "Sequence 01" at 128 BPM.
Unit tests for layout, key order, escapes, parse errors, invalid number text, float formatting against Python repr values, paths, array edits and clone independence.

M2: edit operations in internal/project, matching MpckitT1.xpj / MpckitT3.xpj and rebuilding Demo02.