A tab still slugs the branch: `branch_123` reads `branch-123`, and the case goes too

[blooop]

dl blooop/devlaunch@test/branc-title_123 names the tab
devlaunch@test/branc-title-123. The underscore is now a dash. feat/ABC_123
reads feat/abc-123, so the case goes as well. Measured against main at
0.50.0, by calling WorkspaceId::label directly:

test/branc-title_123 -> id devlaunch-test-branc-title-123-hx1w  label devlaunch@test/branc-title-123
        feat/ABC_123 -> id devlaunch-feat-abc-123-ktsk          label devlaunch@feat/abc-123

Why it happens

Nothing is filtering the title for safety. sanitize_title
(rust/devlaunch-core/src/flows/launch.rs) drops control characters, $, a
backtick and a backslash, and finds nothing to do on this path: a derived name
never holds any of them.

The reshaping is slug
(rust/devlaunch-core/src/domain/workspace_id.rs), which lowercases and
collapses every run of non-alphanumerics to one dash. That rule belongs to the
id, and the id has to obey it: it names a devpod workspace and it is the
container's hostname, so it has to be a DNS label. WorkspaceId::label is
defined as a rendering of the id rather than a second reading of the spec,
which is what keeps a tab matchable by eye against a dl --ls row, so it
inherits the id's alphabet wholesale.

Why that is worth revisiting

A tab is not a DNS label and not a devpod name. #626 made exactly this argument
for the / and put the branch's slashes back, and the same argument covers _,
. and the case: all of them are inert in every sink the label reaches. The OSC
2 escape is BEL-terminated, herdr tab rename takes the label as argv with no
shell, and the container's PS1 line is shell-quoted and re-expanded rather
than re-parsed, so only $, a backtick and a backslash would ever act, and
sanitize_title already drops those.

So #626 was a half-step. It restored one of the four things the id's alphabet
takes off a branch name and left the other three, on no principle that
distinguishes them.

The decision this needs

Not "stop filtering" but what a tab is a rendering of, decided once:

1. The id, as today. The tab and a dl --ls row stay matchable character for
   character apart from the separators. feature/auth and Feature_auth keep
   reading alike, which is the same ambiguity Spell the branch's slashes in the name a tab reads #626 removed for slashes.
2. The branch as typed, cut to the id's budget with the id's
   segment-dropping. A tab then says what you typed, and the price is that the
   readable half of a tab no longer matches the readable half of the id: a
   reader matching devlaunch@Feature_auth against
   devlaunch-feature-auth-3j1t has to know the slug rule to see they are one
   workspace.

Whichever wins, docs/workspaces.md's renderings table is where it is written
down, and the length budget, the dropped middle segments and the four-character
suffix are not in question either way.

Worth pinning in the same change: the label's alphabet is currently asserted in
a_label_holds_one_at_and_nothing_else_a_shell_or_a_terminal_reads
(workspace_id.rs), which would have to move with the decision, and
the_prompt_that_repaints_the_title_renders_it_as_text
(flows/provision.rs) is where a new character earns its keep by being rendered
through a real bash prompt.