fix(pcap): preserve exact UTC capture instants

Request epoch timestamps from capinfos and normalize them once at the
metadata boundary, preserving nanosecond precision and explicit unknowns.
Bound numeric parsing to the producer representation and cover negative
fractions, calendar limits, malformed output and MCP projection.

Adapted from PR #4909. Fixes #4907.

Validation: 47 passed, 24 platform-specific skips across five focused
files; check:fast and test discovery passed. No live Wireshark or
Linux/Windows capture execution was performed on this macOS host.

Co-authored-by: Mert Kaya <47145541+mertkayacs@users.noreply.github.com>

docs/pcap-analysis.md

@@ -58,6 +58,11 @@ tools read that copy, which is hashed again after analysis. Capture metadata and
 literal field values are producer observations; address/protocol counts and DNS
 associations are derivations. Capture Evidence has historical-reference authority.
 
+`capture_start` and `capture_end` are RFC 3339 UTC instants derived from
+`capinfos -S` epoch seconds, retaining all producer fractional digits (up to
+nanoseconds). Empty captures, malformed timestamps and instants outside years
+0000–9999 remain `null`; host-local timestamps without a zone are not inferred.
+
 Addresses count once per packet, including loopback and nested IP occurrences.
 DNS addresses are associated only with one question in the same response packet;
 additional records may be included, so this does not prove final resolution.