update documentation for previous day/intraday gain/loss

docs/guides/read-your-portfolio.md

@@ -24,6 +24,7 @@ ZFIN_HOME=examples/pre-retirement-both zfin portfolio
 Portfolio Summary (examples/pre-retirement-both/portfolio.srf)
 ========================================
   Value: $1,383,137.81  Cost: $658,837.01  Gain/Loss: +$724,300.80 (109.9%)
+  Yesterday: -$1,845.22 (-0.13%)
   Lots: 13 open, 0 closed    Positions: 5 symbols
   Historical:  1M: +3.2%  3M: +13.3%  1Y: +24.5%  3Y: +56.4%  5Y: +56.1%  10Y: +182.6%
 
@@ -37,6 +38,13 @@ How to read it:
 
 - **The header line** is your liquid total, total cost, and aggregate
   gain/loss.
+- **The session line** is a single trading day's move, equities only.
+  It is labelled by the session it actually describes: `Day` when the
+  prices are today's, `Yesterday` when the newest available closes are
+  yesterday's (the usual case mid-session, since this prices from cached
+  closes), or an explicit date after a weekend or with a stale cache.
+  Its percentage is against the previous session's whole account value,
+  so it lines up with the Gain/Loss percentage above.
 - **Historical** is the portfolio's blended price return over trailing
   windows -- a quick "how have my holdings done" gut check.
 - **Each position** shows aggregated shares, average cost, current

docs/reference/cli/portfolio.md

@@ -22,6 +22,7 @@ ZFIN_HOME=examples/pre-retirement-both zfin portfolio
 Portfolio Summary (examples/pre-retirement-both/portfolio.srf)
 ========================================
   Value: $1,383,137.81  Cost: $658,837.01  Gain/Loss: +$724,300.80 (109.9%)
+  Yesterday: -$1,845.22 (-0.13%)
   Lots: 13 open, 0 closed    Positions: 5 symbols
   Historical:  1M: +3.2%  3M: +13.3%  1Y: +24.5%  3Y: +56.4%  5Y: +56.1%  10Y: +182.6%
 
@@ -38,6 +39,42 @@ Portfolio Summary (examples/pre-retirement-both/portfolio.srf)
 
 Manual-priced rows render in warning color (the price may be stale).
 
+### The session-change line
+
+The second line is the move over a single trading session -- a brokerage
+"day change". It covers equity holdings only; cash, CDs and options have
+no daily price move and contribute zero. The percentage is measured
+against the *previous session's whole account value*, cash included, so
+it is comparable with the Gain/Loss percentage above it.
+
+**The label names the session it describes, which is not always today:**
+
+| label | meaning |
+|-------|---------|
+| `Day` | prices are from today -- live quotes, or today's close once it has posted |
+| `Yesterday` | the newest prices available are yesterday's close |
+| `2026-08-21` | older still: a weekend, a holiday, or a stale cache |
+
+So a `zfin portfolio` run during Tuesday's session normally shows
+`Yesterday`, because it prices from cached closes and Tuesday's bar does
+not exist yet. In the TUI's live mode (`L`) it shows `Day`, updating as
+ticks arrive.
+
+A trailing marker appears when a security that *should* have a daily
+price series didn't have one, and names the market value the figure is
+therefore short by:
+
+```
+  Day: +$12,345.67 (+0.85%)  [$1.2M no history]
+```
+
+Holdings you price by hand with `price::` are **not** counted as a gap --
+they can never have a daily series, so flagging them every run would be
+noise. They already render in warning color. The marker is reserved for
+the fixable case: try [`zfin doctor`](doctor.md) or
+[`zfin cache stale`](cache.md) to find out why a security has no candle
+history.
+
 ## See also
 
 - [Read your portfolio](../../guides/read-your-portfolio.md) -- how to interpret this.

docs/reference/tui.md

@@ -74,6 +74,12 @@ price stream (off by default). While on, prices update in place every
 re-values the whole position table and totals as held symbols tick.
 Press `L` again (or switch tabs) to stop.
 
+The Portfolio tab's session-change line follows along: outside live mode
+it reads `Yesterday` (or a date), because prices come from cached closes;
+with the stream on it reads `Day` and tracks today's move as ticks
+arrive. See [`zfin portfolio`](cli/portfolio.md#the-session-change-line)
+for how that figure is defined.
+
 The feed follows
 [`ZFIN_LIVE_QUOTE_PROVIDER`](config/environment.md#live-quotes-and-streaming):
 keyless **Yahoo** (default, ~15-min delayed) or real-time IEX via