feat: add logo + social preview + fix nanosecond RFC 3339 parsing on Python 3.9/3.10

Repo pimpery pass:
- Added centered Pipelock logo to the top of README.
- Added social-preview.png for sharing on link previews. Must be
  uploaded via GitHub repo Settings → Social preview (no REST API
  support for that field).

Correctness fix:
- Python 3.9/3.10 `datetime.fromisoformat` only accepts microsecond
  precision. Go emits nanosecond precision (time.RFC3339Nano).
  `_is_valid_rfc3339` now truncates fractional seconds to 6 digits
  before calling fromisoformat, so Go-emitted timestamps with 7-9
  fractional digits verify correctly on every supported Python
  version. Regex shape-check still covers 1-9 digits, semantic check
  still catches bad dates. Fixes `test_valid_rfc3339_timestamps_accepted[2026-04-09T12:00:00.123456789Z]`.

Author: luckyPipewrench

README.md

@@ -1,3 +1,7 @@
+<p align="center">
+  <img src="assets/pipelock-logo.svg" alt="Pipelock" width="200">
+</p>
+
 # pipelock-verify
 
 [![PyPI](https://img.shields.io/pypi/v/pipelock-verify.svg?label=PyPI&logo=pypi&logoColor=white)](https://pypi.org/project/pipelock-verify/)

assets/pipelock-logo.svg

@@ -64,17 +64,34 @@
 def _is_valid_rfc3339(value: Any) -> bool:
     """Return True if value parses as an RFC 3339 timestamp Go would accept.
 
-    Python's datetime.fromisoformat accepts the ``Z`` suffix natively only
-    on 3.11+, so we substitute it with ``+00:00`` before parsing to support
-    3.9 and 3.10 as well.
+    Go's ``time.RFC3339Nano`` allows up to 9 fractional digits (nanoseconds).
+    Python's ``datetime.fromisoformat`` tops out at microsecond precision
+    (6 fractional digits) and the ``Z`` suffix only parses natively on 3.11+.
+    We handle both differences here so valid Go timestamps verify on 3.9-3.13.
     """
     if not isinstance(value, str):
         return False
     if not _RFC3339_RE.match(value):
         return False
     candidate = value[:-1] + "+00:00" if value.endswith("Z") else value
+    # Truncate fractional seconds to 6 digits so fromisoformat accepts
+    # Go's nanosecond timestamps on Python 3.9/3.10. The regex above has
+    # already validated the overall shape, so we know there is at most one
+    # ``.`` before the timezone offset.
+    match = re.match(
+        r"^(?P<prefix>\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2})"
+        r"(?:\.(?P<frac>\d{1,9}))?"
+        r"(?P<offset>[+-]\d{2}:\d{2})$",
+        candidate,
+    )
+    if match is None:
+        return False
+    prefix = match.group("prefix")
+    frac = match.group("frac") or ""
+    offset = match.group("offset")
+    truncated = f"{prefix}.{frac[:6]}{offset}" if frac else f"{prefix}{offset}"
     try:
-        datetime.fromisoformat(candidate)
+        datetime.fromisoformat(truncated)
     except ValueError:
         return False
     return True