* fix(update): make ensure_uv() survive the update boundary (no first-run crash)
`hermes update` runs the `ensure_uv()` call site from the old, already-imported
`hermes_cli.main` against the *freshly pulled* `managed_uv` (managed_uv is only
ever lazily imported, so it loads from disk post-pull). `ensure_uv()`'s return
arity flipped from a single path string to `(path, fresh_bootstrap)` (4df280d51)
and back to a single string (fb853a178). Installs parked on a 2-tuple release
unpack `uv_bin, fresh_bootstrap = ensure_uv()` against the new single-value
module and crash the first update with
`ValueError: not enough values to unpack (expected 2, got 1)` — inside the
dependency-install step, *before* the PR #39763 subprocess hand-off can run.
Return a `_UvResult` (a `str` subclass) that is usable as the bare path AND
unpackable as `(path|None, fresh_bootstrap)`. Missing uv is `""` (falsy) instead
of `None` so legacy 2-target call sites can unpack a failure without raising,
while `if not uv_bin` keeps working for single-value callers. fresh_bootstrap is
always False (the rebuild-venv path it gated was scrapped in fb853a178).
* docs(update): correct the verified error string + mechanism for ensure_uv()
A hermetic repro (old 2-target call site vs the freshly-pulled single-value
module) shows the first-update crash is exactly the string from PR #39763's
report: `ValueError: too many values to unpack (expected 2)` — not "not enough".
The returned path is a plain `str`, which is iterable, so `uv_bin, fresh =
ensure_uv()` walks its characters; the failure path's `None` return raises
`TypeError: cannot unpack non-iterable NoneType`. Both are fixed by `_UvResult`.
Comment/test wording updated to match; no behavior change.
db204ae203