fix: print user keys that shadow reserved key names as regular fields

[troclaux]

Problem

When using gum log --structured, passing a key named time causes that key-value pair to be silently dropped in the text and JSON formatters.

Reproduction (from charmbracelet/gum#967):

gum log --structured "Test log message" time 0 normal_var 1 time 2
# prints: Test log message normal_var=1
# expected: Test log message time=0 normal_var=1 time=2

The same silent-drop affects any user key whose name matches a built-in key (time, level, caller, prefix) when the value's type does not match the expected built-in type.

Root cause

In textFormatter and jsonFormatterRoot, each built-in key case does a type assertion and renders the value in a special style. When the assertion fails, the code falls through with no output. The logfmtFormatter already handles this correctly by always encoding the key-value regardless of the type assertion result.

Fix

• text.go: Extracts the default KV-writing logic into a writeKVPair helper and calls it as the else branch for TimestampKey, LevelKey, CallerKey, and PrefixKey.
• json.go: Adds an else branch calling the existing jsonFormatterItem helper for the same set of keys.
• text_test.go: Adds TestReservedKeyNonBuiltinValue to cover the reported scenario.

MessageKey is intentionally left unchanged — a user passing msg as a structured key would produce a duplicate message field, which is a separate discussion.