fix(analysis): the review findings - rising gravity, undated readings, the band

A second, deeper review of PR #12 found five things the first review did
not.  All are fixed here.

The band was not the band
-------------------------
The fermentation curve's shaded range was drawn as ONE dataset with
`fill: "+1"`, which fills to the NEXT dataset - and the next dataset was
the median line, not the maximum.  So the band shaded min-to-median, the
upper half of the range was drawn as a hairline but never shaded, and the
tooltip said "range min to max" while the picture showed something else.
The band is now two datasets - the min curve filled down to a max curve -
with the max hidden from the legend.

Rising gravity
--------------
`attenuation_percent` rejected a final gravity below 1.000 and one
implying more than 100% attenuation, but a run whose gravity went UP
produced a NEGATIVE attenuation - which was then averaged into the
headline figure and plotted as a negative bar.  Verified against the live
code: OG 1010 -> FG 1020 gave -100.0%.  Now rejected, and reported by
`implausible_runs` with the reason "the gravity went up, which is usually
a mis-ordered pair of readings".

Undated readings
----------------
An undated reading sorted to the front of the run, so it became the
original gravity - which put the OG after the FG and produced a negative
attenuation.  The OG now comes from the earliest *dated* reading, and
`days_to_target` skips the readings it cannot place on the timeline rather
than giving up on the whole run.  Verified: readings 1050 (dated), 1005
(undated), 1010 (dated) gave None instead of 24 days, and an OG of 1005
instead of 1050.

Active runs on the curve
------------------------
The curve chart filtered on `curve.length > 1` only, so it plotted the
active runs as well - a partial fermentation on a chart meant to describe
how a completed one behaves.  It now excludes them, along with the
suspect runs.

Four fetches, one payload
-------------------------
`/api/runs` carries every reading of every run, and four charts each
fetched it independently - so the analysis page made two identical
round-trips of the whole collection.  There is now one shared fetch.

Also
----
- The "Readings that need a second look" table links to the run, so a
  suspect reading can be corrected in one click.
- `days_to_1015` and the `curves` variable were computed and never used.
  Removed.
- A reading below 1.000 now says what it is - "beer cannot be less dense
  than water, so this is almost certainly a typo (a 1.012 reading entered
  as 012?)".  The reading is still SAVED: rejecting it would lose the
  brewer's actual observation, and seeing it recorded is how they spot the
  mistake.

142 tests (up from 134).

tests/test_app.py

@@ -186,11 +186,16 @@ def test_logout_clears_the_session(logged_in):
 
 
 def test_out_of_range_reading_is_saved_but_warned(logged_in, repo):
+    """A reading outside the range is SAVED with a warning.
+
+    Rejecting it would lose the brewer's actual observation, and seeing it
+    recorded is how they spot and correct a typo.
+    """
     response = logged_in.post("/brews", data={
         "beer_run_id": "12",
         "beer_run_type": "Odd",
         "Date": "2026-05-01",
-        "sg": "950",
+        "sg": "1250",
     }, follow_redirects=True)
     assert response.status_code == 200
     assert b"outside the usual hydrometer range" in response.data
@@ -198,6 +203,25 @@ def test_out_of_range_reading_is_saved_but_warned(logged_in, repo):
     assert repo.readings.find_one({"beer_run_id": "12"}) is not None
 
 
+def test_a_reading_below_water_says_it_is_always_a_typo(logged_in, repo):
+    """A gravity below 1.000 cannot be real, so the warning says so.
+
+    This is the run 12 case: a 1.012 reading entered as "012".  The
+    reading is still saved, but the message names the likely cause.
+    """
+    response = logged_in.post("/brews", data={
+        "beer_run_id": "13",
+        "beer_run_type": "Typo",
+        "Date": "2026-05-01",
+        "sg": "900",
+    }, follow_redirects=True)
+    assert response.status_code == 200
+    assert b"below 1.000" in response.data
+    assert b"typo" in response.data
+    # Still saved, so the mistake is visible and can be corrected.
+    assert repo.readings.find_one({"beer_run_id": "13"}) is not None
+
+
 # ------------------------------------------------------- security: passwords #
 
 def test_legacy_plaintext_password_is_accepted_and_migrated(repo):