fix: unify colour resolution behind a terminal profile

[aymanbagabas]

First of a three PR stack: #84 → #85 → #86.

A screenshot and a color assertion disagreed about what color a cell was, because each resolved palette indices through its own hardcoded table. render/svg.rs had a private Theme where red was #e88388; assert/color.rs had an ANSI16 table where red was #800000. Nothing kept them in sync and nothing noticed they had drifted.

The bug

Same program, same red text, on main:

$ shell-use submit 'printf "\033[31mERROR\033[0m: red text\n"'
$ shell-use screenshot --out shot.svg     # paints ERROR #e88388

$ shell-use expect text ERROR --fg '#800000'   # a color not in the image
$ echo $?
0                                              # ...passes

$ shell-use expect text ERROR --fg '#e88388'   # the color you can see
$ echo $?
1                                              # ...fails

Asserting the color the image actually paints failed, and asserting a color that appears nowhere passed. Either the picture was lying or the assertion was, and a test suite could not tell you which.

before — image paints #e88388	after — image paints #800000

Both tables are deleted. One Colors type resolves every index once, so the renderer and the assertion cannot disagree by construction. --fg '#800000' now passes and matches the picture.

Profiles

The palette is no longer a private constant, so a session can be given one. --config picks the file and --profile the entry; discovery cascades ./shell-use.toml then ~/.shell-use/shell-use.toml.

[profiles.solarized]
scrollback = 10000

[profiles.solarized.colors]
background = "#002b36"
foreground = "#839496"
cursor     = "#d33682"
red        = "#dc322f"
green      = "#859900"
blue       = "#268bd2"

$ shell-use run --config shell-use.toml --profile solarized -- bash

A profile is exactly 19 values: the 16 ANSI colors plus foreground, background, and cursor. It is read only for the life of the session, so a program cannot rewrite the palette a test was pinned against. Anything outside 0-15 comes from a static xterm table, and scrollback defaults to 10000.

Notes for review

• --fg <index> compares the cell's palette index, so it was never affected by this bug and still is not. Only #rrggbb resolves through the palette.
• Snapshots record the slot ("fg": 1), not RGB, so existing baselines survive a palette change. There are tests pinning that.
• Defaults are the VGA palette, foreground #c0c0c0 on background #000000, which is what the assertion side already used, so no assertion changes meaning.

[aymanbagabas]

@cpendery could you take a look? This is the base of a three PR stack:

• fix: unify colour resolution behind a terminal profile #84 (this one) — one palette instead of two that disagreed, plus configurable profiles
• feat: answer OSC colour queries and track dynamic colours #85 — answer OSC color queries, and support setting/resetting colors at runtime
• feat(render): draw the cursor in screenshots #86 — draw the cursor in screenshots

Read them in that order. GitHub could not request you as a reviewer directly (I do not have push access here), hence the mention.