docs(i18n): state the shipped severity for an unresolvable translation key

`content/docs/protocol/kernel/i18n-standard.mdx` promised that an unresolvable
translation-bundle key "is a **warning**, not an error: nothing crashes". Both
halves are false against the shipped linter: `translation-target-unknown` has
carried `severity: 'error'` since #16310, so `os validate`, `os lint` and
`os compile` each exit non-zero on one. An author who read the page and then
wrote a locale key against a target the linter cannot resolve got a failed run,
having been told in writing it would be a nag.

The whole claim is repaired, not just the severity word:

- the severity now reads `error`, names the rule id so it can be grepped, and
  names the three commands that exit non-zero on it;
- the runtime half ("nothing crashes ... renders in its source locale") is kept,
  because it is still true and is precisely the reason the rule gates: the orphan
  key stays a confident-looking grep hit for a surface that was deleted;
- the sibling rule is separated out, since the very next paragraph is about it:
  `translation-option-key-unknown` is still a warning.

The linter is untouched by design. #16310 was a deliberate promotion, so the page
is what drifted.

Claude-Session: https://claude.ai/code/session_017ef78bLdybu3AffehKkhfk
Co-authored-by: Claude <noreply@anthropic.com>

Author: claude

content/docs/protocol/kernel/i18n-standard.mdx

@@ -1024,9 +1024,16 @@ every bundle against the stack it ships with:
 | `dashboards.{dash}` / `.widgets.{id}` / `.actions.{actionUrl}` | the dashboard's `name` / a widget `id` / a header `actionUrl` |
 | `globalActions.{action}` | an action with **no** `objectName` |
 
-An unresolvable key is a **warning**, not an error: nothing crashes, the string
-simply renders in its source locale while every neighbouring label resolves — so
-the app looks translated and one field does not.
+An unresolvable key is an **error**, not a warning: `os validate`, `os lint` and
+`os compile` each report it as `translation-target-unknown` and exit non-zero, so
+a bundle key naming a surface the metadata no longer declares fails the run (it
+was a warning until #16310). Nothing *crashes* — the string simply renders in its
+source locale while every neighbouring label resolves, so the app looks translated
+and one field does not — but that is what earns the error rather than excusing it:
+the orphan key stays a confident-looking grep hit, in every locale, for a surface
+that was deleted. The mis-keyed **option** below is the deliberate exception and
+stays a warning (`translation-option-key-unknown`): it names something real, so
+its remedy is a rename rather than a deletion.
 
 The option case is worth spelling out. Option maps are keyed by the option's
 stored `value`, never by its display label: